[PATCH] X86-64: Add -mskip-rax-setup

[PATCH] X86-64: Add -mskip-rax-setup

[H.J. Lu]

The Linux kernel never passes floating point arguments around, vararg
functions or not. Hence no vector registers are ever used when calling a
vararg function.  But gcc still dutifully emits an "xor %eax,%eax" before
each and every call of a vararg function.  Since no callee use that for
anything, these instructions are redundant.

This patch adds the -mskip-rax-setup option to skip setting up RAX
register when SSE is disabled and there are no variable arguments passed
in vector registers.  Since RAX register is used to avoid unnecessarily
saving vector registers on stack when passing variable arguments, the
impacts of this option are callees may waste some stack space, misbehave
or jump to a random location.  GCC 4.4 or newer don't those issues,
regardless the RAX register value since they don't check the RAX register
value when SSE is disabled, regardless the RAX register value:

https://gcc.gnu.org/ml/gcc-patches/2008-09/msg00127.html

I used it on kernel 3.17.7:

   text	   data	    bss	      dec     hex    filename
11493571  2271232  5926912  19691715 12c78c3 vmlinux.skip-rax
11517879  2271232  5926912  19716023 12cd7b7 vmlinux.orig

It removed 14309 redundant "xor %eax,%eax" instructions and saved about
27KB.  I am currently running the new kernel without any problem.  OK
for trunk?

Thanks.

H.J.
---
gcc/

	* config/i386/i386.c (ix86_expand_call): Skip setting up RAX
	register for -mskip-rax-setup when there are no parameters
	passed in vector registers.
	* config/i386/i386.opt (mskip-rax-setup): New option.
	* doc/invoke.texi: Document -mskip-rax-setup.

gcc/testsuite/

	* gcc.target/i386/amd64-abi-7.c: New tests.
	* gcc.target/i386/amd64-abi-8.c: Likwise.
	* gcc.target/i386/amd64-abi-9.c: Likwise.
---
 gcc/ChangeLog                               |  8 +++++
 gcc/config/i386/i386.c                      |  7 ++++-
 gcc/config/i386/i386.opt                    |  4 +++
 gcc/doc/invoke.texi                         | 13 ++++++++
 gcc/testsuite/ChangeLog                     |  6 ++++
 gcc/testsuite/gcc.target/i386/amd64-abi-7.c | 46 +++++++++++++++++++++++++++++
 gcc/testsuite/gcc.target/i386/amd64-abi-8.c | 18 +++++++++++
 gcc/testsuite/gcc.target/i386/amd64-abi-9.c | 18 +++++++++++
 8 files changed, 119 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/gcc.target/i386/amd64-abi-7.c
 create mode 100644 gcc/testsuite/gcc.target/i386/amd64-abi-8.c
 create mode 100644 gcc/testsuite/gcc.target/i386/amd64-abi-9.c

diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index 24a252a..de7907a 100644
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,3 +1,11 @@
+2014-12-18  H.J. Lu  <hongjiu.lu@intel.com>
+
+	* config/i386/i386.c (ix86_expand_call): Skip setting up RAX
+	register for -mskip-rax-setup when there are no parameters
+	passed in vector registers.
+	* config/i386/i386.opt (mskip-rax-setup): New option.
+	* doc/invoke.texi: Document -mskip-rax-setup.
+
 2014-12-18  Martin Liska  <mliska@suse.cz>
 
 	PR ipa/64146
diff --git a/gcc/config/i386/i386.c b/gcc/config/i386/i386.c
index 17ef751..122a350 100644
--- a/gcc/config/i386/i386.c
+++ b/gcc/config/i386/i386.c
@@ -25461,7 +25461,12 @@ ix86_expand_call (rtx retval, rtx fnaddr, rtx callarg1,
 	}
     }
 
-  if (TARGET_64BIT && INTVAL (callarg2) >= 0)
+  /* Skip setting up RAX register for -mskip-rax-setup when there are no
+     parameters passed in vector registers.  */
+  if (TARGET_64BIT
+      && (INTVAL (callarg2) > 0
+	  || (INTVAL (callarg2) == 0
+	      && (TARGET_SSE || !flag_skip_rax_setup))))
     {
       rtx al = gen_rtx_REG (QImode, AX_REG);
       emit_move_insn (al, callarg2);
diff --git a/gcc/config/i386/i386.opt b/gcc/config/i386/i386.opt
index 3d54bfa..6dc4da2 100644
--- a/gcc/config/i386/i386.opt
+++ b/gcc/config/i386/i386.opt
@@ -831,6 +831,10 @@ Target Report Var(flag_nop_mcount) Init(0)
 Generate mcount/__fentry__ calls as nops. To activate they need to be
 patched in.
 
+mskip-rax-setup
+Target Report Var(flag_skip_rax_setup) Init(0)
+Skip setting up RAX register when passing variable arguments.
+
 m8bit-idiv
 Target Report Mask(USE_8BIT_IDIV) Save
 Expand 32bit/64bit integer divide into 8bit unsigned integer divide with run-time check
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 15068da..33a7ed2 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -16256,6 +16256,19 @@ the profiling functions as nops. This is useful when they
 should be patched in later dynamically. This is likely only
 useful together with @option{-mrecord-mcount}.
 
+@item -mskip-rax-setup
+@itemx -mno-skip-rax-setup
+@opindex mskip-rax-setup
+When generating code for the x86-64 architecture with SSE extensions
+disabled, @option{-skip-rax-setup} can be used to skip setting up RAX
+register when there are no variable arguments passed in vector registers.
+
+@strong{Warning:} Since RAX register is used to avoid unnecessarily
+saving vector registers on stack when passing variable arguments, the
+impacts of this option are callees may waste some stack space,
+misbehave or jump to a random location.  GCC 4.4 or newer don't have
+those issues, regardless the RAX register value.
+
 @item -m8bit-idiv
 @itemx -mno-8bit-idiv
 @opindex 8bit-idiv
diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index 025dfce..6c06503 100644
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,3 +1,9 @@
+2014-12-18  H.J. Lu  <hongjiu.lu@intel.com>
+
+	* gcc.target/i386/amd64-abi-7.c: New tests.
+	* gcc.target/i386/amd64-abi-8.c: Likwise.
+	* gcc.target/i386/amd64-abi-9.c: Likwise.
+
 2014-12-18  Martin Liska  <mliska@suse.cz>
 
 	* g++.dg/ipa/pr64146.C: New test.
diff --git a/gcc/testsuite/gcc.target/i386/amd64-abi-7.c b/gcc/testsuite/gcc.target/i386/amd64-abi-7.c
new file mode 100644
index 0000000..fcca680
--- /dev/null
+++ b/gcc/testsuite/gcc.target/i386/amd64-abi-7.c
@@ -0,0 +1,46 @@
+/* { dg-do run { target { ! { ia32 } } } } */
+/* { dg-options "-O2 -mno-sse" } */
+
+#include <stdarg.h>
+#include <assert.h>
+
+int n1 = 30;
+int n2 = 324;
+void *n3 = (void *) &n2;
+int n4 = 407;
+
+int e1;
+int e2;
+void *e3;
+int e4;
+
+static void
+__attribute__((noinline))
+foo (va_list va_arglist)
+{
+  e2 = va_arg (va_arglist, int);
+  e3 = va_arg (va_arglist, void *);
+  e4 = va_arg (va_arglist, int);
+}
+
+static void
+__attribute__((noinline))
+test (int a1, ...)
+{
+  va_list va_arglist;
+  e1 = a1;
+  va_start (va_arglist, a1);
+  foo (va_arglist);
+  va_end (va_arglist);
+}
+
+int
+main ()
+{
+  test (n1, n2, n3, n4);
+  assert (n1 == e1);
+  assert (n2 == e2);
+  assert (n3 == e3);
+  assert (n4 == e4);
+  return 0;
+}
diff --git a/gcc/testsuite/gcc.target/i386/amd64-abi-8.c b/gcc/testsuite/gcc.target/i386/amd64-abi-8.c
new file mode 100644
index 0000000..b25ceec
--- /dev/null
+++ b/gcc/testsuite/gcc.target/i386/amd64-abi-8.c
@@ -0,0 +1,18 @@
+/* { dg-do compile { target { ! { ia32 } } } } */
+/* { dg-options "-O2 -mno-sse -mskip-rax-setup" } */
+/* { dg-final { scan-assembler-not "xorl\[\\t \]*\\\%eax,\[\\t \]*%eax" } } */
+
+void foo (const char *, ...);
+
+void
+test1 (void)
+{
+  foo ("%d", 20);
+}
+
+int
+test2 (void)
+{
+  foo ("%d", 20);
+  return 3;
+}
diff --git a/gcc/testsuite/gcc.target/i386/amd64-abi-9.c b/gcc/testsuite/gcc.target/i386/amd64-abi-9.c
new file mode 100644
index 0000000..4707eb7
--- /dev/null
+++ b/gcc/testsuite/gcc.target/i386/amd64-abi-9.c
@@ -0,0 +1,18 @@
+/* { dg-do compile { target { ! { ia32 } } } } */
+/* { dg-options "-O2 -mno-sse -mno-skip-rax-setup" } */
+/* { dg-final { scan-assembler-times "xorl\[\\t \]*\\\%eax,\[\\t \]*%eax" 2 } } */
+
+void foo (const char *, ...);
+
+void
+test1 (void)
+{
+  foo ("%d", 20);
+}
+
+int
+test2 (void)
+{
+  foo ("%d", 20);
+  return 3;
+}
-- 
1.9.3

Re: [PATCH] X86-64: Add -mskip-rax-setup

[Uros Bizjak]

On Thu, Dec 18, 2014 at 2:11 PM, H.J. Lu <hongjiu.lu@intel.com> wrote:
> The Linux kernel never passes floating point arguments around, vararg
> functions or not. Hence no vector registers are ever used when calling a
> vararg function.  But gcc still dutifully emits an "xor %eax,%eax" before
> each and every call of a vararg function.  Since no callee use that for
> anything, these instructions are redundant.
>
> This patch adds the -mskip-rax-setup option to skip setting up RAX
> register when SSE is disabled and there are no variable arguments passed
> in vector registers.  Since RAX register is used to avoid unnecessarily
> saving vector registers on stack when passing variable arguments, the
> impacts of this option are callees may waste some stack space, misbehave
> or jump to a random location.  GCC 4.4 or newer don't those issues,
> regardless the RAX register value since they don't check the RAX register
> value when SSE is disabled, regardless the RAX register value:
>
> https://gcc.gnu.org/ml/gcc-patches/2008-09/msg00127.html
>
> I used it on kernel 3.17.7:
>
>    text    data     bss       dec     hex    filename
> 11493571  2271232  5926912  19691715 12c78c3 vmlinux.skip-rax
> 11517879  2271232  5926912  19716023 12cd7b7 vmlinux.orig
>
> It removed 14309 redundant "xor %eax,%eax" instructions and saved about
> 27KB.  I am currently running the new kernel without any problem.  OK
> for trunk?

How about skipping RAX setup unconditionally for !TARGET_SSE? Please
see ix86_conditional_register_usage, where SSE registers are squashed
for !TARGET_SSE, so it is not possible to use them even in the inline
asm.

Uros.

Re: [PATCH] X86-64: Add -mskip-rax-setup

[Uros Bizjak]

On Thu, Dec 18, 2014 at 2:24 PM, Uros Bizjak <ubizjak@gmail.com> wrote:
> On Thu, Dec 18, 2014 at 2:11 PM, H.J. Lu <hongjiu.lu@intel.com> wrote:
>> The Linux kernel never passes floating point arguments around, vararg
>> functions or not. Hence no vector registers are ever used when calling a
>> vararg function.  But gcc still dutifully emits an "xor %eax,%eax" before
>> each and every call of a vararg function.  Since no callee use that for
>> anything, these instructions are redundant.
>>
>> This patch adds the -mskip-rax-setup option to skip setting up RAX
>> register when SSE is disabled and there are no variable arguments passed
>> in vector registers.  Since RAX register is used to avoid unnecessarily
>> saving vector registers on stack when passing variable arguments, the
>> impacts of this option are callees may waste some stack space, misbehave
>> or jump to a random location.  GCC 4.4 or newer don't those issues,
>> regardless the RAX register value since they don't check the RAX register
>> value when SSE is disabled, regardless the RAX register value:
>>
>> https://gcc.gnu.org/ml/gcc-patches/2008-09/msg00127.html
>>
>> I used it on kernel 3.17.7:
>>
>>    text    data     bss       dec     hex    filename
>> 11493571  2271232  5926912  19691715 12c78c3 vmlinux.skip-rax
>> 11517879  2271232  5926912  19716023 12cd7b7 vmlinux.orig
>>
>> It removed 14309 redundant "xor %eax,%eax" instructions and saved about
>> 27KB.  I am currently running the new kernel without any problem.  OK
>> for trunk?
>
> How about skipping RAX setup unconditionally for !TARGET_SSE? Please
> see ix86_conditional_register_usage, where SSE registers are squashed
> for !TARGET_SSE, so it is not possible to use them even in the inline
> asm.

... when -ffreestanding is in effect, of course.

Uros.

Re: [PATCH] X86-64: Add -mskip-rax-setup

[Jakub Jelinek]

On Thu, Dec 18, 2014 at 02:24:06PM +0100, Uros Bizjak wrote:
> > It removed 14309 redundant "xor %eax,%eax" instructions and saved about
> > 27KB.  I am currently running the new kernel without any problem.  OK
> > for trunk?
> 
> How about skipping RAX setup unconditionally for !TARGET_SSE? Please
> see ix86_conditional_register_usage, where SSE registers are squashed
> for !TARGET_SSE, so it is not possible to use them even in the inline
> asm.

I'd say a problem is if a -mno-sse TU calls a vararg function (obviously
it can't pass any float/double arguments) to a function in a TU compiled
with -msse2 or higher where the stdarg pass can't figure out anything, say
#include <stdarg.h>
extern void bar (int, va_list);
void
foo (int x, ...)
{
  va_list ap;
  va_start (ap, x);
  bar (x, ap);
  va_end (ap);
}
If foo is compiled with gcc 4.4 and earlier, it might crash when called from
-mno-sse caller that would not xor %eax,%eax.  If foo is compiled with gcc
4.5? and higher, then it might just randomly save all the xmm registers to
stack (as the test is %al != 0, I think it will be more likely that it will
save it unnecessarily than not).
So I view H.J.'s new option as a user guarantee the callee will be also -mno-sse.

	Jakub

Re: [PATCH] X86-64: Add -mskip-rax-setup

[H.J. Lu]

On Thu, Dec 18, 2014 at 5:51 AM, Jakub Jelinek <jakub@redhat.com> wrote:
> On Thu, Dec 18, 2014 at 02:24:06PM +0100, Uros Bizjak wrote:
>> > It removed 14309 redundant "xor %eax,%eax" instructions and saved about
>> > 27KB.  I am currently running the new kernel without any problem.  OK
>> > for trunk?
>>
>> How about skipping RAX setup unconditionally for !TARGET_SSE? Please
>> see ix86_conditional_register_usage, where SSE registers are squashed
>> for !TARGET_SSE, so it is not possible to use them even in the inline
>> asm.
>
> I'd say a problem is if a -mno-sse TU calls a vararg function (obviously
> it can't pass any float/double arguments) to a function in a TU compiled
> with -msse2 or higher where the stdarg pass can't figure out anything, say
> #include <stdarg.h>
> extern void bar (int, va_list);
> void
> foo (int x, ...)
> {
>   va_list ap;
>   va_start (ap, x);
>   bar (x, ap);
>   va_end (ap);
> }
> If foo is compiled with gcc 4.4 and earlier, it might crash when called from

This was checked into GCC 4.4:

commit d5d9458afdae9056a9624ae9c332dbcdc2f383be
Author: jakub <jakub@138bc75d-0d04-0410-961f-82ee72b054a4>
Date:   Tue Sep 2 19:49:41 2008 +0000

      * config/i386/i386.c (X86_64_VARARGS_SIZE): Removed.
      (setup_incoming_varargs_64): Assume cum != NULL. Set/check
      ix86_varargs_gpr_size and ix86_varargs_fpr_size.  Use
      ix86_varargs_gpr_size instead of X86_64_REGPARM_MAX.
      Don't set ix86_save_varrargs_registers.
      (ix86_setup_incoming_varargs): Assume cum != NULL.
      (ix86_va_start): Check ix86_varargs_gpr_size and
      ix86_varargs_fpr_size instead of cfun->va_list_gpr_size and
      cfun->va_list_fpr_size, respectively.  Subtract 8*X86_64_REGPARM_MAX
      from frame pointer if ix86_varargs_gpr_size == 0.
      (ix86_compute_frame_layout): Updated.
      * config/i386/i386.h (ix86_save_varrargs_registers): Removed.
      (ix86_varargs_gpr_size): Define.
      (ix86_varargs_fpr_size): Likewise.
      (machine_function): Remove save_varrargs_registers.
      Add varargs_gpr_size and varargs_fpr_size.

      * gcc.target/i386/amd64-abi-3.c: New test.
      * gcc.target/i386/amd64-abi-4.c: Likewise.
      * gcc.target/i386/amd64-abi-5.c: Likewise.
      * gcc.target/i386/amd64-abi-6.c: Likewise.


    git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@139910
138bc75d-0d04-0410-961f-82ee72b054a4

> -mno-sse caller that would not xor %eax,%eax.  If foo is compiled with gcc
> 4.5? and higher, then it might just randomly save all the xmm registers to

GCC 4.4 is fine.

> stack (as the test is %al != 0, I think it will be more likely that it will
> save it unnecessarily than not).
> So I view H.J.'s new option as a user guarantee the callee will be also -mno-sse.
>

If foo may be compiled by GCC 4.3 or older, we can't skip setting up
RAX in foo's
callers.  Such an option is useful.


-- 
H.J.

Re: [PATCH] X86-64: Add -mskip-rax-setup

[Uros Bizjak]

On Thu, Dec 18, 2014 at 2:49 PM, Uros Bizjak <ubizjak@gmail.com> wrote:

>>> The Linux kernel never passes floating point arguments around, vararg
>>> functions or not. Hence no vector registers are ever used when calling a
>>> vararg function.  But gcc still dutifully emits an "xor %eax,%eax" before
>>> each and every call of a vararg function.  Since no callee use that for
>>> anything, these instructions are redundant.
>>>
>>> This patch adds the -mskip-rax-setup option to skip setting up RAX
>>> register when SSE is disabled and there are no variable arguments passed
>>> in vector registers.  Since RAX register is used to avoid unnecessarily
>>> saving vector registers on stack when passing variable arguments, the
>>> impacts of this option are callees may waste some stack space, misbehave
>>> or jump to a random location.  GCC 4.4 or newer don't those issues,
>>> regardless the RAX register value since they don't check the RAX register
>>> value when SSE is disabled, regardless the RAX register value:
>>>
>>> https://gcc.gnu.org/ml/gcc-patches/2008-09/msg00127.html
>>>
>>> I used it on kernel 3.17.7:
>>>
>>>    text    data     bss       dec     hex    filename
>>> 11493571  2271232  5926912  19691715 12c78c3 vmlinux.skip-rax
>>> 11517879  2271232  5926912  19716023 12cd7b7 vmlinux.orig
>>>
>>> It removed 14309 redundant "xor %eax,%eax" instructions and saved about
>>> 27KB.  I am currently running the new kernel without any problem.  OK
>>> for trunk?
>>
>> How about skipping RAX setup unconditionally for !TARGET_SSE? Please
>> see ix86_conditional_register_usage, where SSE registers are squashed
>> for !TARGET_SSE, so it is not possible to use them even in the inline
>> asm.
>
> ... when -ffreestanding is in effect, of course.

Ops, this is not the unconditional default kernel compile flag. It is
defined only for 32bit builds, where:

# temporary until string.h is fixed
KBUILD_CFLAGS += -ffreestanding

Yes, it looks to me that new option is the way to go.

Uros.

Re: [PATCH] X86-64: Add -mskip-rax-setup

[H.J. Lu]

On Thu, Dec 18, 2014 at 6:03 AM, Uros Bizjak <ubizjak@gmail.com> wrote:
> On Thu, Dec 18, 2014 at 2:49 PM, Uros Bizjak <ubizjak@gmail.com> wrote:
>
>>>> The Linux kernel never passes floating point arguments around, vararg
>>>> functions or not. Hence no vector registers are ever used when calling a
>>>> vararg function.  But gcc still dutifully emits an "xor %eax,%eax" before
>>>> each and every call of a vararg function.  Since no callee use that for
>>>> anything, these instructions are redundant.
>>>>
>>>> This patch adds the -mskip-rax-setup option to skip setting up RAX
>>>> register when SSE is disabled and there are no variable arguments passed
>>>> in vector registers.  Since RAX register is used to avoid unnecessarily
>>>> saving vector registers on stack when passing variable arguments, the
>>>> impacts of this option are callees may waste some stack space, misbehave
>>>> or jump to a random location.  GCC 4.4 or newer don't those issues,
>>>> regardless the RAX register value since they don't check the RAX register
>>>> value when SSE is disabled, regardless the RAX register value:
>>>>
>>>> https://gcc.gnu.org/ml/gcc-patches/2008-09/msg00127.html
>>>>
>>>> I used it on kernel 3.17.7:
>>>>
>>>>    text    data     bss       dec     hex    filename
>>>> 11493571  2271232  5926912  19691715 12c78c3 vmlinux.skip-rax
>>>> 11517879  2271232  5926912  19716023 12cd7b7 vmlinux.orig
>>>>
>>>> It removed 14309 redundant "xor %eax,%eax" instructions and saved about
>>>> 27KB.  I am currently running the new kernel without any problem.  OK
>>>> for trunk?
>>>
>>> How about skipping RAX setup unconditionally for !TARGET_SSE? Please
>>> see ix86_conditional_register_usage, where SSE registers are squashed
>>> for !TARGET_SSE, so it is not possible to use them even in the inline
>>> asm.
>>
>> ... when -ffreestanding is in effect, of course.
>
> Ops, this is not the unconditional default kernel compile flag. It is
> defined only for 32bit builds, where:
>
> # temporary until string.h is fixed
> KBUILD_CFLAGS += -ffreestanding
>
> Yes, it looks to me that new option is the way to go.

Is this an OK?

Some really old gcc versions used an indirect jump based on the eax input
and they didn't zero extend first.  So with those compilers, you could actually
jump to a random location.  You can enable it only when you can compile
everything with a newer GCC.

Thanks.

-- 
H.J.

Re: [PATCH] X86-64: Add -mskip-rax-setup

[Uros Bizjak]

On Thu, Dec 18, 2014 at 3:09 PM, H.J. Lu <hjl.tools@gmail.com> wrote:

>>>>> The Linux kernel never passes floating point arguments around, vararg
>>>>> functions or not. Hence no vector registers are ever used when calling a
>>>>> vararg function.  But gcc still dutifully emits an "xor %eax,%eax" before
>>>>> each and every call of a vararg function.  Since no callee use that for
>>>>> anything, these instructions are redundant.
>>>>>
>>>>> This patch adds the -mskip-rax-setup option to skip setting up RAX
>>>>> register when SSE is disabled and there are no variable arguments passed
>>>>> in vector registers.  Since RAX register is used to avoid unnecessarily
>>>>> saving vector registers on stack when passing variable arguments, the
>>>>> impacts of this option are callees may waste some stack space, misbehave
>>>>> or jump to a random location.  GCC 4.4 or newer don't those issues,
>>>>> regardless the RAX register value since they don't check the RAX register
>>>>> value when SSE is disabled, regardless the RAX register value:
>>>>>
>>>>> https://gcc.gnu.org/ml/gcc-patches/2008-09/msg00127.html
>>>>>
>>>>> I used it on kernel 3.17.7:
>>>>>
>>>>>    text    data     bss       dec     hex    filename
>>>>> 11493571  2271232  5926912  19691715 12c78c3 vmlinux.skip-rax
>>>>> 11517879  2271232  5926912  19716023 12cd7b7 vmlinux.orig
>>>>>
>>>>> It removed 14309 redundant "xor %eax,%eax" instructions and saved about
>>>>> 27KB.  I am currently running the new kernel without any problem.  OK
>>>>> for trunk?
>>>>
>>>> How about skipping RAX setup unconditionally for !TARGET_SSE? Please
>>>> see ix86_conditional_register_usage, where SSE registers are squashed
>>>> for !TARGET_SSE, so it is not possible to use them even in the inline
>>>> asm.
>>>
>>> ... when -ffreestanding is in effect, of course.
>>
>> Ops, this is not the unconditional default kernel compile flag. It is
>> defined only for 32bit builds, where:
>>
>> # temporary until string.h is fixed
>> KBUILD_CFLAGS += -ffreestanding
>>
>> Yes, it looks to me that new option is the way to go.
>
> Is this an OK?

In principle, I'm OK with the patch approach, but let's wait for
eventual comments from Linux people.

> Some really old gcc versions used an indirect jump based on the eax input
> and they didn't zero extend first.  So with those compilers, you could actually
> jump to a random location.  You can enable it only when you can compile
> everything with a newer GCC.

Uros.

Re: [PATCH] X86-64: Add -mskip-rax-setup

[H. Peter Anvin]

On 12/18/2014 06:12 AM, Uros Bizjak wrote:
>>>
>>> # temporary until string.h is fixed
>>> KBUILD_CFLAGS += -ffreestanding
>>>
>>> Yes, it looks to me that new option is the way to go.
>>
>> Is this an OK?
> 
> In principle, I'm OK with the patch approach, but let's wait for
> eventual comments from Linux people.
> 

Acked-by: H. Peter Anvin <hpa@linux.intel.com>

H.J. already coordinated with us; we are more than happy with this approach.

Thank you!

	-hpa

Re: [PATCH] X86-64: Add -mskip-rax-setup

[H.J. Lu]

On Thu, Dec 18, 2014 at 9:08 AM, H. Peter Anvin <hpa@zytor.com> wrote:
> On 12/18/2014 06:12 AM, Uros Bizjak wrote:
>>>>
>>>> # temporary until string.h is fixed
>>>> KBUILD_CFLAGS += -ffreestanding
>>>>
>>>> Yes, it looks to me that new option is the way to go.
>>>
>>> Is this an OK?
>>
>> In principle, I'm OK with the patch approach, but let's wait for
>> eventual comments from Linux people.
>>
>
> Acked-by: H. Peter Anvin <hpa@linux.intel.com>
>
> H.J. already coordinated with us; we are more than happy with this approach.
>
> Thank you!

I am checking it in now.

Thanks.

-- 
H.J.

Re: [PATCH] X86-64: Add -mskip-rax-setup

[Uros Bizjak]

On Thu, Dec 18, 2014 at 6:08 PM, H. Peter Anvin <hpa@zytor.com> wrote:
> On 12/18/2014 06:12 AM, Uros Bizjak wrote:
>>>>
>>>> # temporary until string.h is fixed
>>>> KBUILD_CFLAGS += -ffreestanding
>>>>
>>>> Yes, it looks to me that new option is the way to go.
>>>
>>> Is this an OK?
>>
>> In principle, I'm OK with the patch approach, but let's wait for
>> eventual comments from Linux people.
>>
>
> Acked-by: H. Peter Anvin <hpa@linux.intel.com>
>
> H.J. already coordinated with us; we are more than happy with this approach.

Great, I'm glad that this specialized option has uses.

Patch is OK.

Thanks,
Uros.

Re: [PATCH] X86-64: Add -mskip-rax-setup

[H. Peter Anvin]

On 12/18/2014 09:43 AM, H.J. Lu wrote:
> 
> Peter, please feel free to use my kernel patch or create a different
> one.
> 

Great, thanks!

	-hpa

Re: [PATCH] X86-64: Add -mskip-rax-setup

[H.J. Lu]

On Thu, Dec 18, 2014 at 9:23 AM, Uros Bizjak <ubizjak@gmail.com> wrote:
> On Thu, Dec 18, 2014 at 6:08 PM, H. Peter Anvin <hpa@zytor.com> wrote:
>> On 12/18/2014 06:12 AM, Uros Bizjak wrote:
>>>>>
>>>>> # temporary until string.h is fixed
>>>>> KBUILD_CFLAGS += -ffreestanding
>>>>>
>>>>> Yes, it looks to me that new option is the way to go.
>>>>
>>>> Is this an OK?
>>>
>>> In principle, I'm OK with the patch approach, but let's wait for
>>> eventual comments from Linux people.
>>>
>>
>> Acked-by: H. Peter Anvin <hpa@linux.intel.com>
>>
>> H.J. already coordinated with us; we are more than happy with this approach.
>
> Great, I'm glad that this specialized option has uses.
>
> Patch is OK.

It is in now.  Here is the GCC 5 change entry.  OK to install?

Peter, please feel free to use my kernel patch or create a different
one.

Thanks.

-- 
H.J.
---
Index: gcc-5/changes.html
===================================================================
RCS file: /cvs/gcc/wwwdocs/htdocs/gcc-5/changes.html,v
retrieving revision 1.52
diff -u -p -r1.52 changes.html
--- gcc-5/changes.html 15 Dec 2014 19:55:08 -0000 1.52
+++ gcc-5/changes.html 18 Dec 2014 17:42:18 -0000
@@ -444,6 +444,10 @@ void operator delete[] (void *, std::siz
  place of the __fentry__ or mcount call, so that a call per function
  can be later patched in. This can be used for low overhead tracing or
  hot code patching.</li>
+ <li>The new <code>-mskip-rax-setup</code> option to skip setting
+ up RAX register when SSE is disabled and there are no variable
+ arguments passed in vector registers.  This can be used to
+ optimize Linux kernel.</li>
   </ul>

 <h3 id="sh">SH</h3>

Re: [PATCH] X86-64: Add -mskip-rax-setup

[Rasmus Villemoes]

On Thu, Dec 18 2014, "H. Peter Anvin" <hpa@zytor.com> wrote:

> On 12/18/2014 09:43 AM, H.J. Lu wrote:
>> 
>> Peter, please feel free to use my kernel patch or create a different
>> one.
>> 
>
> Great, thanks!
>

Thanks for accepting this idea, and especially to H.J. for already
having done all the work.

Minor thing: If it's not too late, I'd appreciate a 'Suggested-by' or
similar mention in the kernel change log.

Thanks,
Rasmus

Re: [PATCH] X86-64: Add -mskip-rax-setup

[H. Peter Anvin]

On 12/18/2014 10:37 AM, Rasmus Villemoes wrote:
> 
> Minor thing: If it's not too late, I'd appreciate a 'Suggested-by' or
> similar mention in the kernel change log.
> 

I think we can get that.

	-hpa

Re: [PATCH] X86-64: Add -mskip-rax-setup

[Mike Stump]

On Dec 18, 2014, at 9:43 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
> + arguments passed in vector registers.  This can be used to
> + optimize Linux kernel.</li>

English, to optimize _the_ Linux kernel?

Re: [PATCH] X86-64: Add -mskip-rax-setup

[H.J. Lu]

On Thu, Dec 18, 2014 at 11:22 AM, Mike Stump <mikestump@comcast.net> wrote:
> On Dec 18, 2014, at 9:43 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
>> + arguments passed in vector registers.  This can be used to
>> + optimize Linux kernel.</li>
>
> English, to optimize _the_ Linux kernel?

Updated.

-- 
H.J.
---
Index: gcc-5/changes.html
===================================================================
RCS file: /cvs/gcc/wwwdocs/htdocs/gcc-5/changes.html,v
retrieving revision 1.52
diff -u -p -r1.52 changes.html
--- gcc-5/changes.html 15 Dec 2014 19:55:08 -0000 1.52
+++ gcc-5/changes.html 18 Dec 2014 19:35:28 -0000
@@ -444,6 +444,10 @@ void operator delete[] (void *, std::siz
  place of the __fentry__ or mcount call, so that a call per function
  can be later patched in. This can be used for low overhead tracing or
  hot code patching.</li>
+ <li>The new <code>-mskip-rax-setup</code> option to skip setting
+ up RAX register when SSE is disabled and there are no variable
+ arguments passed in vector registers.  This can be used to
+ optimize the Linux kernel.</li>
   </ul>

 <h3 id="sh">SH</h3>

Re: [PATCH] X86-64: Add -mskip-rax-setup

[Gerald Pfeifer]

On Thursday 2014-12-18 11:35, H.J. Lu wrote:
> Updated.

"the RAX register" (i.e., add "the"), and I suggest to make
this a sentence, similar to my previous mail for the other
update.

This is fine with these changes.

Thank you -- and that's a nice hack (in the positive sense)!

Gerald

Re: [PATCH] X86-64: Add -mskip-rax-setup

[H.J. Lu]

On Thu, Dec 25, 2014 at 9:07 PM, Gerald Pfeifer <gerald@pfeifer.com> wrote:
> On Thursday 2014-12-18 11:35, H.J. Lu wrote:
>> Updated.
>
> "the RAX register" (i.e., add "the"), and I suggest to make
> this a sentence, similar to my previous mail for the other
> update.
>
> This is fine with these changes.

Done.

> Thank you -- and that's a nice hack (in the positive sense)!
>

It really is a hack :-) and was suggested by Rasmus Villemoes.

Thanks.

-- 
H.J.

Re: [PATCH] X86-64: Add -mskip-rax-setup

[Mike Stump]

On Dec 26, 2014, at 7:57 AM, "H.J. Lu" <hjl.tools@gmail.com> wrote:
> On Thu, Dec 25, 2014 at 9:07 PM, Gerald Pfeifer <gerald@pfeifer.com> wrote:
>> On Thursday 2014-12-18 11:35, H.J. Lu wrote:
>>> Updated.
>> 
>> "the RAX register" (i.e., add "the"), and I suggest to make
>> this a sentence, similar to my previous mail for the other
>> update.
>> 
>> This is fine with these changes.
> 
> Done.
> 
>> Thank you -- and that's a nice hack (in the positive sense)!
>> 
> 
> It really is a hack :-) and was suggested by Rasmus Villemoes.

So, my suggestion is similar, but different.  Imagine you find 10 of these things over the years, 10 flags look horrible.  I like the -mkernel style flag where all are bundled up under it, essentially it becomes the kernel API flag.

Re: [PATCH] X86-64: Add -mskip-rax-setup

[H.J. Lu]

On Sat, Dec 27, 2014 at 8:18 AM, Mike Stump <mrs@mrs.kithrup.com> wrote:
> On Dec 26, 2014, at 7:57 AM, "H.J. Lu" <hjl.tools@gmail.com> wrote:
>> On Thu, Dec 25, 2014 at 9:07 PM, Gerald Pfeifer <gerald@pfeifer.com> wrote:
>>> On Thursday 2014-12-18 11:35, H.J. Lu wrote:
>>>> Updated.
>>>
>>> "the RAX register" (i.e., add "the"), and I suggest to make
>>> this a sentence, similar to my previous mail for the other
>>> update.
>>>
>>> This is fine with these changes.
>>
>> Done.
>>
>>> Thank you -- and that's a nice hack (in the positive sense)!
>>>
>>
>> It really is a hack :-) and was suggested by Rasmus Villemoes.
>
> So, my suggestion is similar, but different.  Imagine you find 10 of these things over the years, 10 flags look horrible.  I like the -mkernel style flag where all are bundled up under it, essentially it becomes the kernel API flag.

So far, for Linux x86-64 kernel, we added -mskip-rax-setup and allowed
-mpreferred-stack-boundary=3.  We will see how far it goes :-).


-- 
H.J.