* [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
end of thread, other threads:[~2023-02-02 5:50 UTC | newest] Thread overview: 4+ messages (download: mbox.gz / follow: Atom feed) -- links below jump to the message on this page -- 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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