Generate commented assembly with --emit=asm #35741
Comments
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There might be a |
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This flag is already added, should be usable (issue can be closed?): |
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Any reason not to make this the default? |
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Likely the fact that it may change the behavior (I don't actually know what that means but this seems like a valid assumption). |
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it looks like -Z asm-comments doesn't do this. MCOptions.AsmVerbose must be set to true for the assembler to emit them. |
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So it looks like -Zasm-comments isn't actually used anywhere based on what I can find in the source... or, rather, its uses are in methods that are themselves never called. |
Mark-Simulacrum
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A-inline-assembly
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Yeah, to get comments I believe I needed to set a verbose flag in some llvm context. Clang does this by default and it would be nice if rustc did too. |
Mark-Simulacrum
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the
C-feature-request
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IIUC without this neither the playground nor godbolt (compiler-explorer) can map assembly to lines of Rust source code. |
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Godbolt uses debug info AFAIK? Our debug info isn't always great but in general it's working, isn't it? |
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It seems like they just don't do it much. Godbolt uses |
Make LLVM emit assembly comments with -Z asm-comments Fixes rust-lang#35741, and makes `-Z asm-comments` actually do something useful. Before: ``` .section .text.main,"ax",@progbits .globl main .p2align 4, 0x90 .type main,@function main: .cfi_startproc pushq %rax .cfi_def_cfa_offset 16 movslq %edi, %rax leaq _ZN1t4main17he95a7d4f1843730eE(%rip), %rdi movq %rsi, (%rsp) movq %rax, %rsi movq (%rsp), %rdx callq _ZN3std2rt10lang_start17h3121da83b2bc3697E movl %eax, %ecx movl %ecx, %eax popq %rcx .cfi_def_cfa_offset 8 retq .Lfunc_end8: .size main, .Lfunc_end8-main .cfi_endproc ``` After: ``` .section .text.main,"ax",@progbits .globl main # -- Begin function main .p2align 4, 0x90 .type main,@function main: # @main .cfi_startproc pushq %rax .cfi_def_cfa_offset 16 movslq %edi, %rax leaq _ZN1t4main17he95a7d4f1843730eE(%rip), %rdi movq %rsi, (%rsp) # 8-byte Spill movq %rax, %rsi movq (%rsp), %rdx # 8-byte Reload callq _ZN3std2rt10lang_start17h3121da83b2bc3697E movl %eax, %ecx movl %ecx, %eax popq %rcx .cfi_def_cfa_offset 8 retq .Lfunc_end8: .size main, .Lfunc_end8-main .cfi_endproc # -- End function ```
Make LLVM emit assembly comments with -Z asm-comments Fixes rust-lang#35741, and makes `-Z asm-comments` actually do something useful. Before: ``` .section .text.main,"ax",@progbits .globl main .p2align 4, 0x90 .type main,@function main: .cfi_startproc pushq %rax .cfi_def_cfa_offset 16 movslq %edi, %rax leaq _ZN1t4main17he95a7d4f1843730eE(%rip), %rdi movq %rsi, (%rsp) movq %rax, %rsi movq (%rsp), %rdx callq _ZN3std2rt10lang_start17h3121da83b2bc3697E movl %eax, %ecx movl %ecx, %eax popq %rcx .cfi_def_cfa_offset 8 retq .Lfunc_end8: .size main, .Lfunc_end8-main .cfi_endproc ``` After: ``` .section .text.main,"ax",@progbits .globl main # -- Begin function main .p2align 4, 0x90 .type main,@function main: # @main .cfi_startproc # %bb.0: pushq %rax .cfi_def_cfa_offset 16 movslq %edi, %rax leaq _ZN1t4main17he95a7d4f1843730eE(%rip), %rdi movq %rsi, (%rsp) # 8-byte Spill movq %rax, %rsi movq (%rsp), %rdx # 8-byte Reload callq _ZN3std2rt10lang_start17h3121da83b2bc3697E movl %eax, %ecx movl %ecx, %eax popq %rcx .cfi_def_cfa_offset 8 retq .Lfunc_end8: .size main, .Lfunc_end8-main .cfi_endproc # -- End function ```
Make LLVM emit assembly comments with -Z asm-comments Fixes rust-lang#35741, and makes `-Z asm-comments` actually do something useful. Before: ``` .section .text.main,"ax",@progbits .globl main .p2align 4, 0x90 .type main,@function main: .cfi_startproc pushq %rax .cfi_def_cfa_offset 16 movslq %edi, %rax leaq _ZN1t4main17he95a7d4f1843730eE(%rip), %rdi movq %rsi, (%rsp) movq %rax, %rsi movq (%rsp), %rdx callq _ZN3std2rt10lang_start17h3121da83b2bc3697E movl %eax, %ecx movl %ecx, %eax popq %rcx .cfi_def_cfa_offset 8 retq .Lfunc_end8: .size main, .Lfunc_end8-main .cfi_endproc ``` After: ``` .section .text.main,"ax",@progbits .globl main # -- Begin function main .p2align 4, 0x90 .type main,@function main: # @main .cfi_startproc # %bb.0: pushq %rax .cfi_def_cfa_offset 16 movslq %edi, %rax leaq _ZN1t4main17he95a7d4f1843730eE(%rip), %rdi movq %rsi, (%rsp) # 8-byte Spill movq %rax, %rsi movq (%rsp), %rdx # 8-byte Reload callq _ZN3std2rt10lang_start17h3121da83b2bc3697E movl %eax, %ecx movl %ecx, %eax popq %rcx .cfi_def_cfa_offset 8 retq .Lfunc_end8: .size main, .Lfunc_end8-main .cfi_endproc # -- End function ```
clang -S produces nicely commented .s files. It would be nice if rustc did this too.
e.g. simd shuffles are commented
.loc directives have the corresponding line number comment
.loc 1 4 25 is_stmt 1 ## test.c:4:25
debug information is commented
.byte 14 ## DW_FORM_strp
.byte 17 ## DW_AT_low_pc
.byte 1 ## DW_FORM_addr
.byte 18 ## DW_AT_high_pc
.byte 1 ## DW_FORM_addr
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