* Turn on -fomit-frame-pointer by default for 32bit x86?
@ 2010-07-13 18:15 H.J. Lu
2010-07-13 20:40 ` Richard Guenther
2010-07-16 19:35 ` Andi Kleen
0 siblings, 2 replies; 12+ messages in thread
From: H.J. Lu @ 2010-07-13 18:15 UTC (permalink / raw)
To: GCC Patches, Uros Bizjak
Hi,
I checked the default code for 32bit. We currently generate
[hjl@gnu-6 tmp]$ cat x.c
void
foo ()
{
}
[hjl@gnu-6 tmp]$ /usr/gcc-4.5/bin/gcc -m32 -S x.c -O3
[hjl@gnu-6 tmp]$ cat x.s
.file "x.c"
.text
.globl foo
.type foo, @function
foo:
pushl %ebp
movl %esp, %ebp
popl %ebp
ret
Can we turn on -fomit-frame-pointer by default for 32bit?
--
H.J.
^ permalink raw reply [flat|nested] 12+ messages in thread* Re: Turn on -fomit-frame-pointer by default for 32bit x86? 2010-07-13 18:15 Turn on -fomit-frame-pointer by default for 32bit x86? H.J. Lu @ 2010-07-13 20:40 ` Richard Guenther 2010-07-13 20:59 ` H.J. Lu 2010-07-16 19:35 ` Andi Kleen 1 sibling, 1 reply; 12+ messages in thread From: Richard Guenther @ 2010-07-13 20:40 UTC (permalink / raw) To: H.J. Lu; +Cc: GCC Patches, Uros Bizjak On Tue, Jul 13, 2010 at 8:15 PM, H.J. Lu <hjl.tools@gmail.com> wrote: > Hi, > > I checked the default code for 32bit. We currently generate > > [hjl@gnu-6 tmp]$ cat x.c > void > foo () > { > } > [hjl@gnu-6 tmp]$ /usr/gcc-4.5/bin/gcc -m32 -S x.c -O3 > [hjl@gnu-6 tmp]$ cat x.s > .file "x.c" > .text > .globl foo > .type foo, @function > foo: > pushl %ebp > movl %esp, %ebp > popl %ebp > ret > > Can we turn on -fomit-frame-pointer by default for 32bit? No, we don't have unwind information by default for 32bit. Richard. > > -- > H.J. > ^ permalink raw reply [flat|nested] 12+ messages in thread
* Re: Turn on -fomit-frame-pointer by default for 32bit x86? 2010-07-13 20:40 ` Richard Guenther @ 2010-07-13 20:59 ` H.J. Lu 2010-07-13 21:02 ` Andrew Pinski 0 siblings, 1 reply; 12+ messages in thread From: H.J. Lu @ 2010-07-13 20:59 UTC (permalink / raw) To: Richard Guenther; +Cc: GCC Patches, Uros Bizjak On Tue, Jul 13, 2010 at 1:40 PM, Richard Guenther <richard.guenther@gmail.com> wrote: > On Tue, Jul 13, 2010 at 8:15 PM, H.J. Lu <hjl.tools@gmail.com> wrote: >> Hi, >> >> I checked the default code for 32bit. We currently generate >> >> [hjl@gnu-6 tmp]$ cat x.c >> void >> foo () >> { >> } >> [hjl@gnu-6 tmp]$ /usr/gcc-4.5/bin/gcc -m32 -S x.c -O3 >> [hjl@gnu-6 tmp]$ cat x.s >> .file "x.c" >> .text >> .globl foo >> .type foo, @function >> foo: >> pushl %ebp >> movl %esp, %ebp >> popl %ebp >> ret >> >> Can we turn on -fomit-frame-pointer by default for 32bit? > > No, we don't have unwind information by default for 32bit. > What will stop working when -fomit-frame-pointer is on? -- H.J. ^ permalink raw reply [flat|nested] 12+ messages in thread
* Re: Turn on -fomit-frame-pointer by default for 32bit x86? 2010-07-13 20:59 ` H.J. Lu @ 2010-07-13 21:02 ` Andrew Pinski 2010-07-13 21:03 ` Andrew Pinski 0 siblings, 1 reply; 12+ messages in thread From: Andrew Pinski @ 2010-07-13 21:02 UTC (permalink / raw) To: H.J. Lu; +Cc: Richard Guenther, GCC Patches, Uros Bizjak On Tue, Jul 13, 2010 at 1:59 PM, H.J. Lu <hjl.tools@gmail.com> wrote: > What will stop working when -fomit-frame-pointer is on? backtraces when debugging information is not turned on. -- Pinski ^ permalink raw reply [flat|nested] 12+ messages in thread
* Re: Turn on -fomit-frame-pointer by default for 32bit x86? 2010-07-13 21:02 ` Andrew Pinski @ 2010-07-13 21:03 ` Andrew Pinski 2010-07-13 21:44 ` H.J. Lu 0 siblings, 1 reply; 12+ messages in thread From: Andrew Pinski @ 2010-07-13 21:03 UTC (permalink / raw) To: H.J. Lu; +Cc: Richard Guenther, GCC Patches, Uros Bizjak On Tue, Jul 13, 2010 at 2:01 PM, Andrew Pinski <pinskia@gmail.com> wrote: > On Tue, Jul 13, 2010 at 1:59 PM, H.J. Lu <hjl.tools@gmail.com> wrote: >> What will stop working when -fomit-frame-pointer is on? > > backtraces when debugging information is not turned on. See http://gcc.gnu.org/ml/gcc-patches/2004-08/msg01033.html and many more. This is not the first time this has been discussed. ^ permalink raw reply [flat|nested] 12+ messages in thread
* Re: Turn on -fomit-frame-pointer by default for 32bit x86? 2010-07-13 21:03 ` Andrew Pinski @ 2010-07-13 21:44 ` H.J. Lu 2010-07-14 8:09 ` Richard Guenther 2010-07-14 12:00 ` Dave Korn 0 siblings, 2 replies; 12+ messages in thread From: H.J. Lu @ 2010-07-13 21:44 UTC (permalink / raw) To: Andrew Pinski; +Cc: Richard Guenther, GCC Patches, Uros Bizjak On Tue, Jul 13, 2010 at 2:02 PM, Andrew Pinski <pinskia@gmail.com> wrote: > On Tue, Jul 13, 2010 at 2:01 PM, Andrew Pinski <pinskia@gmail.com> wrote: >> On Tue, Jul 13, 2010 at 1:59 PM, H.J. Lu <hjl.tools@gmail.com> wrote: >>> What will stop working when -fomit-frame-pointer is on? >> >> backtraces when debugging information is not turned on. > > See http://gcc.gnu.org/ml/gcc-patches/2004-08/msg01033.html and many > more. This is not the first time this has been discussed. Most of 32bit x86 assembly codes in glibc have .eh_frame section. If backtrace is absolutely needed, they can add -fasynchronous-unwind-tables or -fno-omit-frame-pointer. -- H.J. ^ permalink raw reply [flat|nested] 12+ messages in thread
* Re: Turn on -fomit-frame-pointer by default for 32bit x86? 2010-07-13 21:44 ` H.J. Lu @ 2010-07-14 8:09 ` Richard Guenther 2010-07-14 13:47 ` H.J. Lu 2010-07-14 12:00 ` Dave Korn 1 sibling, 1 reply; 12+ messages in thread From: Richard Guenther @ 2010-07-14 8:09 UTC (permalink / raw) To: H.J. Lu; +Cc: Andrew Pinski, GCC Patches, Uros Bizjak On Tue, Jul 13, 2010 at 11:44 PM, H.J. Lu <hjl.tools@gmail.com> wrote: > On Tue, Jul 13, 2010 at 2:02 PM, Andrew Pinski <pinskia@gmail.com> wrote: >> On Tue, Jul 13, 2010 at 2:01 PM, Andrew Pinski <pinskia@gmail.com> wrote: >>> On Tue, Jul 13, 2010 at 1:59 PM, H.J. Lu <hjl.tools@gmail.com> wrote: >>>> What will stop working when -fomit-frame-pointer is on? >>> >>> backtraces when debugging information is not turned on. >> >> See http://gcc.gnu.org/ml/gcc-patches/2004-08/msg01033.html and many >> more. This is not the first time this has been discussed. > > Most of 32bit x86 assembly codes in glibc have .eh_frame section. > If backtrace is absolutely needed, they can add -fasynchronous-unwind-tables > or -fno-omit-frame-pointer. We build opensuse with -fomit-frame-pointer -fasynchronous-unwind-tables. If you want to make -fomit-frame-pointer the default then you should enable unwind tables by default. Richard. > > -- > H.J. > ^ permalink raw reply [flat|nested] 12+ messages in thread
* Re: Turn on -fomit-frame-pointer by default for 32bit x86? 2010-07-14 8:09 ` Richard Guenther @ 2010-07-14 13:47 ` H.J. Lu 2010-07-14 21:55 ` H.J. Lu 0 siblings, 1 reply; 12+ messages in thread From: H.J. Lu @ 2010-07-14 13:47 UTC (permalink / raw) To: Richard Guenther; +Cc: Andrew Pinski, GCC Patches, Uros Bizjak On Wed, Jul 14, 2010 at 1:09 AM, Richard Guenther <richard.guenther@gmail.com> wrote: > On Tue, Jul 13, 2010 at 11:44 PM, H.J. Lu <hjl.tools@gmail.com> wrote: >> On Tue, Jul 13, 2010 at 2:02 PM, Andrew Pinski <pinskia@gmail.com> wrote: >>> On Tue, Jul 13, 2010 at 2:01 PM, Andrew Pinski <pinskia@gmail.com> wrote: >>>> On Tue, Jul 13, 2010 at 1:59 PM, H.J. Lu <hjl.tools@gmail.com> wrote: >>>>> What will stop working when -fomit-frame-pointer is on? >>>> >>>> backtraces when debugging information is not turned on. >>> >>> See http://gcc.gnu.org/ml/gcc-patches/2004-08/msg01033.html and many >>> more. This is not the first time this has been discussed. >> >> Most of 32bit x86 assembly codes in glibc have .eh_frame section. >> If backtrace is absolutely needed, they can add -fasynchronous-unwind-tables >> or -fno-omit-frame-pointer. > > We build opensuse with -fomit-frame-pointer -fasynchronous-unwind-tables. > If you want to make -fomit-frame-pointer the default then you should enable > unwind tables by default. > I will try and make it target/OS dependent. -- H.J. ^ permalink raw reply [flat|nested] 12+ messages in thread
* Re: Turn on -fomit-frame-pointer by default for 32bit x86? 2010-07-14 13:47 ` H.J. Lu @ 2010-07-14 21:55 ` H.J. Lu 0 siblings, 0 replies; 12+ messages in thread From: H.J. Lu @ 2010-07-14 21:55 UTC (permalink / raw) To: Richard Guenther; +Cc: Andrew Pinski, GCC Patches, Uros Bizjak On Wed, Jul 14, 2010 at 6:46 AM, H.J. Lu <hjl.tools@gmail.com> wrote: > On Wed, Jul 14, 2010 at 1:09 AM, Richard Guenther > <richard.guenther@gmail.com> wrote: >> On Tue, Jul 13, 2010 at 11:44 PM, H.J. Lu <hjl.tools@gmail.com> wrote: >>> On Tue, Jul 13, 2010 at 2:02 PM, Andrew Pinski <pinskia@gmail.com> wrote: >>>> On Tue, Jul 13, 2010 at 2:01 PM, Andrew Pinski <pinskia@gmail.com> wrote: >>>>> On Tue, Jul 13, 2010 at 1:59 PM, H.J. Lu <hjl.tools@gmail.com> wrote: >>>>>> What will stop working when -fomit-frame-pointer is on? >>>>> >>>>> backtraces when debugging information is not turned on. >>>> >>>> See http://gcc.gnu.org/ml/gcc-patches/2004-08/msg01033.html and many >>>> more. This is not the first time this has been discussed. >>> >>> Most of 32bit x86 assembly codes in glibc have .eh_frame section. >>> If backtrace is absolutely needed, they can add -fasynchronous-unwind-tables >>> or -fno-omit-frame-pointer. >> >> We build opensuse with -fomit-frame-pointer -fasynchronous-unwind-tables. >> If you want to make -fomit-frame-pointer the default then you should enable >> unwind tables by default. >> > > I will try and make it target/OS dependent. > Here is a patch. How does it look? -- H.J. -- diff --git a/gcc/config/i386/i386.c b/gcc/config/i386/i386.c index 4fd2aab..fe30bfd 100644 --- a/gcc/config/i386/i386.c +++ b/gcc/config/i386/i386.c @@ -2985,10 +2985,23 @@ override_options (bool main_args_p) { if (flag_zee == 2) flag_zee = 0; + /* Unwind info is not correct around the CFG unless either a + frame pointer is present or -maccumulate-outgoing-args is + set. When both -fasynchronous-unwind-tables and + -fomit-frame-pointer are turned on by default, turn off + both if -mno-accumulate-outgoing-args is used. */ if (flag_omit_frame_pointer == 2) - flag_omit_frame_pointer = 0; + flag_omit_frame_pointer + = (TARGET_SUBTARGET32_OMIT_FRAME_POINTER_DEFAULT + && (!TARGET_SUBTARGET32_ASYNCHRONOUS_UNWIND_TABLES_DEFAULT + || !(target_flags_explicit + & MASK_ACCUMULATE_OUTGOING_ARGS))); if (flag_asynchronous_unwind_tables == 2) - flag_asynchronous_unwind_tables = 0; + flag_asynchronous_unwind_tables + = (TARGET_SUBTARGET32_ASYNCHRONOUS_UNWIND_TABLES_DEFAULT + && (!TARGET_SUBTARGET32_OMIT_FRAME_POINTER_DEFAULT + || !(target_flags_explicit + & MASK_ACCUMULATE_OUTGOING_ARGS))); if (flag_pcc_struct_return == 2) flag_pcc_struct_return = DEFAULT_PCC_STRUCT_RETURN; } diff --git a/gcc/config/i386/i386.h b/gcc/config/i386/i386.h index eb3eb9f..c0ae95f 100644 --- a/gcc/config/i386/i386.h +++ b/gcc/config/i386/i386.h @@ -467,6 +467,8 @@ extern tree x86_mfence; /* Extra bits to force on w/ 32-bit mode. */ #define TARGET_SUBTARGET32_DEFAULT 0 #define TARGET_SUBTARGET32_ISA_DEFAULT 0 +#define TARGET_SUBTARGET32_OMIT_FRAME_POINTER_DEFAULT 0 +#define TARGET_SUBTARGET32_ASYNCHRONOUS_UNWIND_TABLES_DEFAULT 0 /* Extra bits to force on w/ 64-bit mode. */ #define TARGET_SUBTARGET64_DEFAULT 0 diff --git a/gcc/config/i386/linux.h b/gcc/config/i386/linux.h index 81dfd1e..61d53b5 100644 --- a/gcc/config/i386/linux.h +++ b/gcc/config/i386/linux.h @@ -219,3 +219,10 @@ along with GCC; see the file COPYING3. If not see /* i386 glibc provides __stack_chk_guard in %gs:0x14. */ #define TARGET_THREAD_SSP_OFFSET 0x14 #endif + +/* Turn on -fomit-frame-pointer and -fasynchronous-unwind-tables by + default. */ +#undef TARGET_SUBTARGET32_OMIT_FRAME_POINTER_DEFAULT +#define TARGET_SUBTARGET32_OMIT_FRAME_POINTER_DEFAULT 1 +#undef TARGET_SUBTARGET32_ASYNCHRONOUS_UNWIND_TABLES_DEFAULT +#define TARGET_SUBTARGET32_ASYNCHRONOUS_UNWIND_TABLES_DEFAULT 1 diff --git a/gcc/config/i386/linux64.h b/gcc/config/i386/linux64.h index 33b4dc9..5a02205 100644 --- a/gcc/config/i386/linux64.h +++ b/gcc/config/i386/linux64.h @@ -123,3 +123,10 @@ see the files COPYING3 and COPYING.RUNTIME respectively. If not, see x86_64 glibc provides it in %fs:0x28. */ #define TARGET_THREAD_SSP_OFFSET (TARGET_64BIT ? 0x28 : 0x14) #endif + +/* Turn on -fomit-frame-pointer and -fasynchronous-unwind-tables by + default. */ +#undef TARGET_SUBTARGET32_OMIT_FRAME_POINTER_DEFAULT +#define TARGET_SUBTARGET32_OMIT_FRAME_POINTER_DEFAULT 1 +#undef TARGET_SUBTARGET32_ASYNCHRONOUS_UNWIND_TABLES_DEFAULT +#define TARGET_SUBTARGET32_ASYNCHRONOUS_UNWIND_TABLES_DEFAULT 1 ^ permalink raw reply [flat|nested] 12+ messages in thread
* Re: Turn on -fomit-frame-pointer by default for 32bit x86? 2010-07-13 21:44 ` H.J. Lu 2010-07-14 8:09 ` Richard Guenther @ 2010-07-14 12:00 ` Dave Korn 1 sibling, 0 replies; 12+ messages in thread From: Dave Korn @ 2010-07-14 12:00 UTC (permalink / raw) To: H.J. Lu; +Cc: Andrew Pinski, Richard Guenther, GCC Patches, Uros Bizjak On 13/07/2010 22:44, H.J. Lu wrote: > On Tue, Jul 13, 2010 at 2:02 PM, Andrew Pinski <pinskia@gmail.com> wrote: >> On Tue, Jul 13, 2010 at 2:01 PM, Andrew Pinski <pinskia@gmail.com> wrote: >>> On Tue, Jul 13, 2010 at 1:59 PM, H.J. Lu <hjl.tools@gmail.com> wrote: >>>> What will stop working when -fomit-frame-pointer is on? >>> backtraces when debugging information is not turned on. >> See http://gcc.gnu.org/ml/gcc-patches/2004-08/msg01033.html and many >> more. This is not the first time this has been discussed. > > Most of 32bit x86 assembly codes in glibc have .eh_frame section. > If backtrace is absolutely needed, they can add -fasynchronous-unwind-tables > or -fno-omit-frame-pointer. > Alas not all the x86 world is a glibc. So if you want to do this it has to be target-dependent I think. cheers, DaveK ^ permalink raw reply [flat|nested] 12+ messages in thread
* Re: Turn on -fomit-frame-pointer by default for 32bit x86? 2010-07-13 18:15 Turn on -fomit-frame-pointer by default for 32bit x86? H.J. Lu 2010-07-13 20:40 ` Richard Guenther @ 2010-07-16 19:35 ` Andi Kleen 2010-07-16 19:43 ` Jeff Law 1 sibling, 1 reply; 12+ messages in thread From: Andi Kleen @ 2010-07-16 19:35 UTC (permalink / raw) To: H.J. Lu; +Cc: GCC Patches, Uros Bizjak "H.J. Lu" <hjl.tools@gmail.com> writes: > [hjl@gnu-6 tmp]$ /usr/gcc-4.5/bin/gcc -m32 -S x.c -O3 > [hjl@gnu-6 tmp]$ cat x.s > .file "x.c" > .text > .globl foo > .type foo, @function > foo: > pushl %ebp > movl %esp, %ebp > popl %ebp > ret > > Can we turn on -fomit-frame-pointer by default for 32bit? I think it's a good idea, but in general one side effect one should be aware of is that it breaks many profilers with calltrace per sample support. Those tend to not be able to read unwind tables. -Andi -- ak@linux.intel.com -- Speaking for myself only. ^ permalink raw reply [flat|nested] 12+ messages in thread
* Re: Turn on -fomit-frame-pointer by default for 32bit x86? 2010-07-16 19:35 ` Andi Kleen @ 2010-07-16 19:43 ` Jeff Law 0 siblings, 0 replies; 12+ messages in thread From: Jeff Law @ 2010-07-16 19:43 UTC (permalink / raw) To: Andi Kleen; +Cc: H.J. Lu, GCC Patches, Uros Bizjak On 07/16/10 13:35, Andi Kleen wrote: > "H.J. Lu"<hjl.tools@gmail.com> writes: > > >> [hjl@gnu-6 tmp]$ /usr/gcc-4.5/bin/gcc -m32 -S x.c -O3 >> [hjl@gnu-6 tmp]$ cat x.s >> .file "x.c" >> .text >> .globl foo >> .type foo, @function >> foo: >> pushl %ebp >> movl %esp, %ebp >> popl %ebp >> ret >> >> Can we turn on -fomit-frame-pointer by default for 32bit? >> > I think it's a good idea, but in general one side effect one should > be aware of is that it breaks many profilers with calltrace per sample > support. Those tend to not be able to read unwind tables. > Speaking of, I sent you a message on fentry about a month ago, but didn't get a reply: I was ready to commit, except that we get a few new testsuite failures which I think touch on the issue of implicit setting of -fomit-frame-pointer issue. See: /* Set the default values for switches whose default depends on TARGET_64BIT in case they weren't overwritten by command line options. */ if (TARGET_64BIT) { /* Mach-O doesn't support omitting the frame pointer for now. */ if (flag_omit_frame_pointer == 2) flag_omit_frame_pointer = (TARGET_MACHO ? 0 : 1); if (flag_asynchronous_unwind_tables == 2) flag_asynchronous_unwind_tables = 1; if (flag_pcc_struct_return == 2) flag_pcc_struct_return = 0; } else { if (flag_omit_frame_pointer == 2) flag_omit_frame_pointer = 0; if (flag_asynchronous_unwind_tables == 2) flag_asynchronous_unwind_tables = 0; if (flag_pcc_struct_return == 2) flag_pcc_struct_return = DEFAULT_PCC_STRUCT_RETURN; } For x86_64, -fomit-frame-pointer is implicitly set; therefore with your patch someone compiling with -O2 -pg will get a diagnostic informing them those options are incompatible except with -mfentry is added. I don't think you intended to get a diagnostic in this case. It's certainly a behaviour change. Jeff ^ permalink raw reply [flat|nested] 12+ messages in thread
end of thread, other threads:[~2010-07-16 19:43 UTC | newest] Thread overview: 12+ messages (download: mbox.gz / follow: Atom feed) -- links below jump to the message on this page -- 2010-07-13 18:15 Turn on -fomit-frame-pointer by default for 32bit x86? H.J. Lu 2010-07-13 20:40 ` Richard Guenther 2010-07-13 20:59 ` H.J. Lu 2010-07-13 21:02 ` Andrew Pinski 2010-07-13 21:03 ` Andrew Pinski 2010-07-13 21:44 ` H.J. Lu 2010-07-14 8:09 ` Richard Guenther 2010-07-14 13:47 ` H.J. Lu 2010-07-14 21:55 ` H.J. Lu 2010-07-14 12:00 ` Dave Korn 2010-07-16 19:35 ` Andi Kleen 2010-07-16 19:43 ` Jeff Law
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