[Bug rtl-optimization/61605] New: Potential optimization: Keep unclobbered argument registers live across function calls

[Bug rtl-optimization/61605] New: Potential optimization: Keep unclobbered argument registers live across function calls

[patrick at parcs dot ath.cx]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61605

            Bug ID: 61605
           Summary: Potential optimization: Keep unclobbered argument
                    registers live across function calls
           Product: gcc
           Version: 4.10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: patrick at parcs dot ath.cx

If a function is known to not clobber an argument register then the caller
shouldn't have to save/reload that register across the function call.  Example:

The C code:

static int __attribute__ ((noinline))
bar (int x)
{
  return x + 3;
}

int
foo (int y)
{
  return y + bar (y);
}

generates:

bar:
.LFB0:
        .cfi_startproc
        leal    3(%rdi), %eax
        ret
        .cfi_endproc
foo:
.LFB1:
        .cfi_startproc
        movl    %edi, %edx
        call    bar
        addl    %edx, %eax
        ret
        .cfi_endproc

Here, %edi is saved to %edx even though the call to bar doesn't clobber %edi. 
The assembly for the function foo could potentially be:

        call    bar
        addl    %edi, %eax

[Bug rtl-optimization/61605] Potential optimization: Keep unclobbered argument registers live across function calls

[andi-gcc at firstfloor dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61605

Andi Kleen <andi-gcc at firstfloor dot org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |andi-gcc at firstfloor dot org,
                   |                            |tom at codesourcery dot com

--- Comment #1 from Andi Kleen <andi-gcc at firstfloor dot org> ---
This is in theory implemented in mainline with -fuse-caller-save
It doesn't seem to work for me though. I also didn't see the option doing
anything on a larger program.

[Bug rtl-optimization/61605] Potential optimization: Keep unclobbered argument registers live across function calls

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61605

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Target|                            |x86_64-*-*

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Andi Kleen from comment #1)
> This is in theory implemented in mainline with -fuse-caller-save
> It doesn't seem to work for me though. I also didn't see the option doing
> anything on a larger program.

Most likely because it is not fully implemented for x86 :).

[Bug rtl-optimization/61605] Potential optimization: Keep unclobbered argument registers live across function calls

[andi-gcc at firstfloor dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61605

--- Comment #3 from Andi Kleen <andi-gcc at firstfloor dot org> ---
It was supposed to be enabled with 

Date:   Fri May 30 11:39:49 2014 +0000

    -fuse-caller-save - Enable for i386

    2014-05-30  Tom de Vries  <tom@codesourcery.com>

        * config/i386/i386.c (TARGET_CALL_FUSAGE_CONTAINS_NON_CALLEE_CLOBBERS):
        Redefine as true.

        * gcc.target/i386/fuse-caller-save.c: New test.
        * gcc.dg/ira-shrinkwrap-prep-1.c: Run with -fno-use-caller-save.
        * gcc.dg/ira-shrinkwrap-prep-2.c: Same.

    git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@211078
138bc75d-0d04-0410-961f-82ee72b054a4

[Bug rtl-optimization/61605] Potential optimization: Keep unclobbered argument registers live across function calls

[vries at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61605

vries at gcc dot gnu.org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2014-09-29
                 CC|                            |vries at gcc dot gnu.org
     Ever confirmed|0                           |1

--- Comment #4 from vries at gcc dot gnu.org ---
> If a function is known to not clobber an argument register then the caller
> shouldn't have to save/reload that register across the function call.

If a function is known to not clobber an call_used_reg then the caller
can use it as a non-call_used_reg across the function call.

This diff shows the example with -fno-use-caller-save vs -fuse-caller-save:
...
 foo:
 .LFB1:
     .cfi_startproc
-    pushq    %rbx
-    .cfi_def_cfa_offset 16
-    .cfi_offset 3, -16
-    movl    %edi, %ebx
+    movl    %edi, %edx
     call    bar
-    addl    %ebx, %eax
-    popq    %rbx
-    .cfi_def_cfa_offset 8
+    addl    %edx, %eax
     ret
     .cfi_endproc
 .LFE1:
...
-fuse-caller-save removes the entry/exit save/restore pair
'pushq %rbx'/'popq %rbx'.

The 'movl %edi, %edx' is indeed non-optimal, but it's not a 'save' in the sense
of save/restore pair generated at function entry/exit or around function calls.
It's a copy at function entry of a hard reg argumument to a pseudo reg,
generated at expand, which is followed by a copy of the pseudo reg to the same
register to set the argument for the function call:
...
(insn 2 4 3 2 (set (reg/v:SI 86 [ yD.1755 ])
        (reg:SI 5 di [ yD.1755 ])) test.c:9 -1
     (nil))
(note 3 2 6 2 NOTE_INSN_FUNCTION_BEG)
(insn 6 3 7 2 (set (reg:SI 5 di)
        (reg/v:SI 86 [ yD.1755 ])) test.c:10 -1
     (nil))
...
The second insn is removed in pass_fast_rtl_dce. The reg-alloc choiche for
pseudo 86 in the first insn is dx, and the insn remains.

I think there could be two ways to address this:
1. Teach a pass after ira, like pass_cprop_hardreg or pass_gcse2 to use the
   information collected by fuse-calller-save.
2. Teach ira to prefer the dx to di in this case.

My guess would be pass_cprop_hardreg.

[Bug rtl-optimization/61605] Potential optimization: Keep unclobbered argument registers live across function calls

[vries at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61605

--- Comment #6 from vries at gcc dot gnu.org ---
Created attachment 33618
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=33618&action=edit
[1/2] Use fuse-caller-save-info in cprop-hardreg

[Bug rtl-optimization/61605] Potential optimization: Keep unclobbered argument registers live across function calls

[vries at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61605

--- Comment #7 from vries at gcc dot gnu.org ---
Created attachment 33619
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=33619&action=edit
[2/2] Don't regard a copy with identical src and dest as killing dest

This patch adds handling of copies with identical source and destination
register in copyprop_hardreg_forward_1.

Using this patch series we get the desired code.

[Bug rtl-optimization/61605] Potential optimization: Keep unclobbered argument registers live across function calls

[vries at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61605

vries at gcc dot gnu.org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |patch

--- Comment #8 from vries at gcc dot gnu.org ---
Patches submitted at:
https://gcc.gnu.org/ml/gcc-patches/2014-10/msg01513.html
https://gcc.gnu.org/ml/gcc-patches/2014-10/msg01514.html

[Bug rtl-optimization/61605] Potential optimization: Keep unclobbered argument registers live across function calls

[vries at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61605

--- Comment #10 from vries at gcc dot gnu.org ---
Author: vries
Date: Fri Oct 17 06:36:45 2014
New Revision: 216365

URL: https://gcc.gnu.org/viewcvs?rev=216365&root=gcc&view=rev
Log:
Use fuse-caller-save info in cprop-hardreg

2014-10-17  Tom de Vries  <tom@codesourcery.com>

    PR rtl-optimization/61605
    * regcprop.c (copyprop_hardreg_forward_1): Use
    regs_invalidated_by_this_call instead of regs_invalidated_by_call.

    * gcc.target/i386/fuse-caller-save.c: Update addition check.  Add movl
    absence check.

Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/regcprop.c
    trunk/gcc/testsuite/ChangeLog
    trunk/gcc/testsuite/gcc.target/i386/fuse-caller-save.c

[Bug rtl-optimization/61605] Potential optimization: Keep unclobbered argument registers live across function calls

[vries at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61605

--- Comment #9 from vries at gcc dot gnu.org ---
Author: vries
Date: Fri Oct 17 06:36:35 2014
New Revision: 216364

URL: https://gcc.gnu.org/viewcvs?rev=216364&root=gcc&view=rev
Log:
Handle copy cycles in pass_cprop_hardreg

2014-10-17  Tom de Vries  <tom@codesourcery.com>

    PR rtl-optimization/61605
    * regcprop.c (copyprop_hardreg_forward_1): Add copy_p and noop_p.  Don't
    notice stores for noops.  Don't regard noops as copies.

Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/regcprop.c