On Thu, Jul 17, 2025 at 11:22 PM H.J. Lu <hjl.to...@gmail.com> wrote:
>
> For TLS calls:
>
> 1. UNSPEC_TLS_GD:
>
> (parallel [
> (set (reg:DI 0 ax)
> (call:DI (mem:QI (symbol_ref:DI ("__tls_get_addr")))
> (const_int 0 [0])))
> (unspec:DI [(symbol_ref:DI ("e") [flags 0x50])
> (reg/f:DI 7 sp)] UNSPEC_TLS_GD)
> (clobber (reg:DI 5 di))])
>
> 2. UNSPEC_TLS_LD_BASE:
>
> (parallel [
> (set (reg:DI 0 ax)
> (call:DI (mem:QI (symbol_ref:DI ("__tls_get_addr")))
> (const_int 0 [0])))
> (unspec:DI [(reg/f:DI 7 sp)] UNSPEC_TLS_LD_BASE)])
>
> 3. UNSPEC_TLSDESC:
>
> (parallel [
> (set (reg/f:DI 104)
> (plus:DI (unspec:DI [
> (symbol_ref:DI ("_TLS_MODULE_BASE_") [flags 0x10])
> (reg:DI 114)
> (reg/f:DI 7 sp)] UNSPEC_TLSDESC)
> (const:DI (unspec:DI [
> (symbol_ref:DI ("e") [flags 0x1a])
> ] UNSPEC_DTPOFF))))
> (clobber (reg:CC 17 flags))])
>
> (parallel [
> (set (reg:DI 101)
> (unspec:DI [(symbol_ref:DI ("e") [flags 0x50])
> (reg:DI 112)
> (reg/f:DI 7 sp)] UNSPEC_TLSDESC))
> (clobber (reg:CC 17 flags))])
>
> they return the same value for the same input value. But multiple calls
> with the same input value may be generated for simple programs like:
>
> void a(long *);
> int b(void);
> void c(void);
> static __thread long e;
> long
> d(void)
> {
> a(&e);
> if (b())
> c();
> return e;
> }
>
> When compiled with -O2 -fPIC -mtls-dialect=gnu2, the following codes are
> generated:
>
> .type d, @function
> d:
> .LFB0:
> .cfi_startproc
> pushq %rbx
> .cfi_def_cfa_offset 16
> .cfi_offset 3, -16
> leaq e@TLSDESC(%rip), %rbx
> movq %rbx, %rax
> call *e@TLSCALL(%rax)
> addq %fs:0, %rax
> movq %rax, %rdi
> call a@PLT
> call b@PLT
> testl %eax, %eax
> jne .L8
> movq %rbx, %rax
> call *e@TLSCALL(%rax)
> popq %rbx
> .cfi_remember_state
> .cfi_def_cfa_offset 8
> movq %fs:(%rax), %rax
> ret
> .p2align 4,,10
> .p2align 3
> .L8:
> .cfi_restore_state
> call c@PLT
> movq %rbx, %rax
> call *e@TLSCALL(%rax)
> popq %rbx
> .cfi_def_cfa_offset 8
> movq %fs:(%rax), %rax
> ret
> .cfi_endproc
>
> There are 3 "call *e@TLSCALL(%rax)". They all return the same value.
> Rename the remove_redundant_vector pass to the x86_cse pass, for 64bit,
> extend it to also remove redundant TLS calls to generate:
>
> d:
> .LFB0:
> .cfi_startproc
> pushq %rbx
> .cfi_def_cfa_offset 16
> .cfi_offset 3, -16
> leaq e@TLSDESC(%rip), %rax
> movq %fs:0, %rdi
> call *e@TLSCALL(%rax)
> addq %rax, %rdi
> movq %rax, %rbx
> call a@PLT
> call b@PLT
> testl %eax, %eax
> jne .L8
> movq %fs:(%rbx), %rax
> popq %rbx
> .cfi_remember_state
> .cfi_def_cfa_offset 8
> ret
> .p2align 4,,10
> .p2align 3
> .L8:
> .cfi_restore_state
> call c@PLT
> movq %fs:(%rbx), %rax
> popq %rbx
> .cfi_def_cfa_offset 8
> ret
> .cfi_endproc
>
> with only one "call *e@TLSCALL(%rax)". This reduces the number of
> __tls_get_addr calls in libgcc.a by 72%:
>
> __tls_get_addr calls before after
> libgcc.a 868 243
>
> gcc/
>
> PR target/81501
> * config/i386/i386-features.cc (x86_cse_kind): Add X86_CSE_TLS_GD,
> X86_CSE_TLS_LD_BASE and X86_CSE_TLSDESC.
> (redundant_load): Renamed to ...
> (redundant_pattern): This.
> (replace_tls_call): New.
> (ix86_place_single_tls_call): Likewise.
> (pass_remove_redundant_vector_load): Renamed to ...
> (pass_x86_cse): This. Add val, def_insn, mode, scalar_mode,
> kind, candidate_kind, x86_cse, candidate_gnu_tls_p,
> candidate_gnu2_tls_p and candidate_vector_p.
> (pass_x86_cse::candidate_gnu_tls_p): New.
> (pass_x86_cse::candidate_gnu2_tls_p): Likewise.
Can we define insn attribute for those tls patterns, so that we can
just check the attribute instead of going through each rtx to check
for the pattern.
> (pass_x86_cse::candidate_vector_p): Likewise.
> (remove_redundant_vector_load): Renamed to ...
> (pass_x86_cse::x86_cse): This. Extend to remove redundant TLS
> calls.
> (make_pass_remove_redundant_vector_load): Renamed to ...
> (make_pass_x86_cse): This.
> (config/i386/i386-passes.def): Replace
> pass_remove_redundant_vector_load with pass_x86_cse.
> config/i386/i386-protos.h (ix86_tls_get_addr): New.
> (make_pass_remove_redundant_vector_load): Renamed to ...
> (make_pass_x86_cse): This.
> * config/i386/i386.cc (ix86_tls_get_addr): Remove static.
> * config/i386/i386.h (machine_function): Add
> tls_descriptor_call_multiple_p.
> * config/i386/i386.md (@tls_global_dynamic_64_<mode>): Set
> tls_descriptor_call_multiple_p.
> (@tls_local_dynamic_base_64_<mode>): Likewise.
> (@tls_dynamic_gnu2_64_<mode>): Likewise.
> (*tls_dynamic_gnu2_lea_64_<mode>): Renamed to ...
> (tls_dynamic_gnu2_lea_64_<mode>): This.
> (*tls_dynamic_gnu2_call_64_<mode>): Renamed to ...
> (tls_dynamic_gnu2_call_64_<mode>): This.
> (*tls_dynamic_gnu2_combine_64_<mode>): Renamed to ...
> (tls_dynamic_gnu2_combine_64_<mode>): This.
>
--
BR,
Hongtao
