* [PATCH] x86: Use DW_EH_PE_indirect|DW_EH_PE_pcrel encodings for -fno-pic code
@ 2023-02-01 7:38 Fangrui Song
2023-02-01 9:01 ` Jakub Jelinek
0 siblings, 1 reply; 4+ messages in thread
From: Fangrui Song @ 2023-02-01 7:38 UTC (permalink / raw)
To: gcc-patches; +Cc: Fangrui Song
Follow aarch64 and riscv ports by using (global ? DW_EH_PE_indirect : 0)
| DW_EH_PE_pcrel for -fno-pic code. This avoids a canonical PLT entry
for a personality reference in .eh_frame and a copy relocation for a
typeinfo reference in .gcc_except_table, when the definition is in
libstdc++.so.6.
gcc/
PR target/108622
* config/i386/i386.cc (asm_preferred_eh_data_format): Always use the
PIC encoding. Use DW_EH_PE_indirect if global.
---
gcc/config/i386/i386.cc | 23 +++++++----------------
1 file changed, 7 insertions(+), 16 deletions(-)
diff --git a/gcc/config/i386/i386.cc b/gcc/config/i386/i386.cc
index 3cacf738c4a..b572dd46a62 100644
--- a/gcc/config/i386/i386.cc
+++ b/gcc/config/i386/i386.cc
@@ -22851,22 +22851,13 @@ ix86_stack_protect_fail (void)
int
asm_preferred_eh_data_format (int code, int global)
{
- /* PE-COFF is effectively always -fPIC because of the .reloc section. */
- if (flag_pic || TARGET_PECOFF || !ix86_direct_extern_access)
- {
- int type = DW_EH_PE_sdata8;
- if (ptr_mode == SImode
- || ix86_cmodel == CM_SMALL_PIC
- || (ix86_cmodel == CM_MEDIUM_PIC && (global || code)))
- type = DW_EH_PE_sdata4;
- return (global ? DW_EH_PE_indirect : 0) | DW_EH_PE_pcrel | type;
- }
-
- if (ix86_cmodel == CM_SMALL
- || (ix86_cmodel == CM_MEDIUM && code))
- return DW_EH_PE_udata4;
-
- return DW_EH_PE_absptr;
+ int type = DW_EH_PE_sdata8;
+ if (ptr_mode == SImode || ix86_cmodel == CM_SMALL ||
+ ix86_cmodel == CM_SMALL_PIC ||
+ (ix86_cmodel == CM_MEDIUM && code) ||
+ (ix86_cmodel == CM_MEDIUM_PIC && (global || code)))
+ type = DW_EH_PE_sdata4;
+ return (global ? DW_EH_PE_indirect : 0) | DW_EH_PE_pcrel | type;
}
\f
/* Implement targetm.vectorize.builtin_vectorization_cost. */
--
2.39.1.456.gfc5497dd1b-goog
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: [PATCH] x86: Use DW_EH_PE_indirect|DW_EH_PE_pcrel encodings for -fno-pic code
2023-02-01 7:38 [PATCH] x86: Use DW_EH_PE_indirect|DW_EH_PE_pcrel encodings for -fno-pic code Fangrui Song
@ 2023-02-01 9:01 ` Jakub Jelinek
2023-02-01 9:09 ` Jakub Jelinek
0 siblings, 1 reply; 4+ messages in thread
From: Jakub Jelinek @ 2023-02-01 9:01 UTC (permalink / raw)
To: Fangrui Song; +Cc: gcc-patches
On Tue, Jan 31, 2023 at 11:38:59PM -0800, Fangrui Song via Gcc-patches wrote:
> Follow aarch64 and riscv ports by using (global ? DW_EH_PE_indirect : 0)
> | DW_EH_PE_pcrel for -fno-pic code. This avoids a canonical PLT entry
> for a personality reference in .eh_frame and a copy relocation for a
> typeinfo reference in .gcc_except_table, when the definition is in
> libstdc++.so.6.
>
> gcc/
>
> PR target/108622
> * config/i386/i386.cc (asm_preferred_eh_data_format): Always use the
> PIC encoding. Use DW_EH_PE_indirect if global.
Not a review, just comments:
> diff --git a/gcc/config/i386/i386.cc b/gcc/config/i386/i386.cc
> index 3cacf738c4a..b572dd46a62 100644
> --- a/gcc/config/i386/i386.cc
> +++ b/gcc/config/i386/i386.cc
> @@ -22851,22 +22851,13 @@ ix86_stack_protect_fail (void)
> int
> asm_preferred_eh_data_format (int code, int global)
> {
> - /* PE-COFF is effectively always -fPIC because of the .reloc section. */
> - if (flag_pic || TARGET_PECOFF || !ix86_direct_extern_access)
> - {
> - int type = DW_EH_PE_sdata8;
> - if (ptr_mode == SImode
> - || ix86_cmodel == CM_SMALL_PIC
> - || (ix86_cmodel == CM_MEDIUM_PIC && (global || code)))
> - type = DW_EH_PE_sdata4;
> - return (global ? DW_EH_PE_indirect : 0) | DW_EH_PE_pcrel | type;
> - }
> -
> - if (ix86_cmodel == CM_SMALL
> - || (ix86_cmodel == CM_MEDIUM && code))
> - return DW_EH_PE_udata4;
> -
> - return DW_EH_PE_absptr;
> + int type = DW_EH_PE_sdata8;
> + if (ptr_mode == SImode || ix86_cmodel == CM_SMALL ||
> + ix86_cmodel == CM_SMALL_PIC ||
> + (ix86_cmodel == CM_MEDIUM && code) ||
> + (ix86_cmodel == CM_MEDIUM_PIC && (global || code)))
The above is incorrectly formatted, the GCC Coding Conventions
say || etc. shouldn't be at the end of lines, but rather at the
start of the next ones.
And, while I can understand the rationale for global cases
(though am not sure I agree, as currently the user can choose
by using -mno-direct-extern-access or not using it, the above change
disallows the choice), for !global I miss the point altogether.
In non-PIC (and non-PECOFF maybe, don't know that format well)
code if code references a local symbol, it will never generate
a copy relocation and so DW_EH_PE_udata4 could be just fine and cheaper
to handle on the consumer sides.
Jakub
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: [PATCH] x86: Use DW_EH_PE_indirect|DW_EH_PE_pcrel encodings for -fno-pic code
2023-02-01 9:01 ` Jakub Jelinek
@ 2023-02-01 9:09 ` Jakub Jelinek
2023-02-02 5:50 ` Fangrui Song
0 siblings, 1 reply; 4+ messages in thread
From: Jakub Jelinek @ 2023-02-01 9:09 UTC (permalink / raw)
To: Fangrui Song, gcc-patches
On Wed, Feb 01, 2023 at 10:01:36AM +0100, Jakub Jelinek via Gcc-patches wrote:
> And, while I can understand the rationale for global cases
> (though am not sure I agree, as currently the user can choose
> by using -mno-direct-extern-access or not using it, the above change
> disallows the choice), for !global I miss the point altogether.
> In non-PIC (and non-PECOFF maybe, don't know that format well)
> code if code references a local symbol, it will never generate
> a copy relocation and so DW_EH_PE_udata4 could be just fine and cheaper
> to handle on the consumer sides.
Plus, e.g. for the typeinfo references, if using the default
-mdirect-extern-access and RTTI is used, there will be copy relocations
anyway and so the .eh_frame growth will be just waste of .got space.
Jakub
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: [PATCH] x86: Use DW_EH_PE_indirect|DW_EH_PE_pcrel encodings for -fno-pic code
2023-02-01 9:09 ` Jakub Jelinek
@ 2023-02-02 5:50 ` Fangrui Song
0 siblings, 0 replies; 4+ messages in thread
From: Fangrui Song @ 2023-02-02 5:50 UTC (permalink / raw)
To: Jakub Jelinek; +Cc: gcc-patches
> The above is incorrectly formatted, the GCC Coding Conventions
say || etc. shouldn't be at the end of lines, but rather at the
start of the next ones.
Ack.
> And, while I can understand the rationale for global cases
> (though am not sure I agree, as currently the user can choose
> by using -mno-direct-extern-access or not using it, the above change
> disallows the choice), for !global I miss the point altogether.
> In non-PIC (and non-PECOFF maybe, don't know that format well)
> code if code references a local symbol, it will never generate
> a copy relocation and so DW_EH_PE_udata4 could be just fine and cheaper
> to handle on the consumer sides.
On Wed, Feb 1, 2023 at 1:09 AM Jakub Jelinek <jakub@redhat.com> wrote:
>
> On Wed, Feb 01, 2023 at 10:01:36AM +0100, Jakub Jelinek via Gcc-patches wrote:
> > And, while I can understand the rationale for global cases
> > (though am not sure I agree, as currently the user can choose
> > by using -mno-direct-extern-access or not using it, the above change
> > disallows the choice), for !global I miss the point altogether.
> > In non-PIC (and non-PECOFF maybe, don't know that format well)
> > code if code references a local symbol, it will never generate
> > a copy relocation and so DW_EH_PE_udata4 could be just fine and cheaper
> > to handle on the consumer sides.
>
> Plus, e.g. for the typeinfo references, if using the default
> -mdirect-extern-access and RTTI is used, there will be copy relocations
> anyway and so the .eh_frame growth will be just waste of .got space.
>
> Jakub
>
There is a copy relocation but the canonical PLT entry for
__gxx_personality_v0 can be removed.
-fno-pic C++ exception code is less common nowadays, so there seems little value
to keep a complex condition (if (flag_pic || TARGET_PECOFF ||
!ix86_direct_extern_access)) to
optimize a bit for the object file size (I think you mean .data space
instead of .got space).
The .eh_frame growth is nearly negligible as well.
^ permalink raw reply [flat|nested] 4+ messages in thread
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2023-02-01 7:38 [PATCH] x86: Use DW_EH_PE_indirect|DW_EH_PE_pcrel encodings for -fno-pic code Fangrui Song
2023-02-01 9:01 ` Jakub Jelinek
2023-02-01 9:09 ` Jakub Jelinek
2023-02-02 5:50 ` Fangrui Song
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