Re: [PATCH] RFC: -fasm-show-source

[Prathamesh Kulkarni <prathamesh dot kulkarni at linaro dot org>]

On 12 August 2016 at 02:04, David Malcolm <dmalcolm@redhat.com> wrote:
> I sometimes find myself scouring assembler output from the compiler
> and trying to figure out which instructions correspond to which
> lines of source code; I believe this is a common activity for some
> end-users.
Hi David,
I usually use gcc -g -Wa,-adhln to trace C source from the assembly.
I tried your example and it gave more or less a similar output (attached).
However the source mapping with your patch looks better.

Thanks,
Prathamesh
>
> The following patch adds a new -fasm-show-source option, which
> emits comments into the generated asm showing the pertinent
> line of source code, whenever it changes.  It uses the same logic
> as debug_hooks->source_line for tracking this (for handling
> line-based breakpoints).
>
> An example can be seen in the invoke.texi part of the patch.  As
> noted there, it's aimed at end-users, rather than gcc developers.
> The example shows a relatively short function; the option is
> likely to be much more useful for longer functions.
>
> I think it would further improve usability if this option were enabled
> by default when the final output is .s (either via -S, or by "-o foo.s").
> Ideas on how to implement that (in the driver) would be welcome - I
> started looking at the spec-handling code, but thought I'd post the
> idea here first, before diving in too deeply.
>
> Successfully bootstrapped&regrtested on x86_64-pc-linux-gnu; adds
> 2 PASS results to gcc.sum.
>
> Thoughts?  OK for trunk as-is?
>
> gcc/ChangeLog:
>         * common.opt (fasm-show-source): New option.
>         * doc/invoke.texi (Code Generation Options): Add
>         -fasm-show-source.
>         (-fasm-show-source): New item.
>         * final.c (asm_show_source): New function.
>         (final_scan_insn): Call asm_show_source.
>
> gcc/testsuite/ChangeLog:
>         * gcc.dg/fasm-show-source-1.c: New test case.
> ---
>  gcc/common.opt                            |  5 +++
>  gcc/doc/invoke.texi                       | 71 ++++++++++++++++++++++++++++++-
>  gcc/final.c                               | 29 ++++++++++++-
>  gcc/testsuite/gcc.dg/fasm-show-source-1.c | 15 +++++++
>  4 files changed, 117 insertions(+), 3 deletions(-)
>  create mode 100644 gcc/testsuite/gcc.dg/fasm-show-source-1.c
>
> diff --git a/gcc/common.opt b/gcc/common.opt
> index 8a292ed..56ce513 100644
> --- a/gcc/common.opt
> +++ b/gcc/common.opt
> @@ -939,6 +939,11 @@ fargument-noalias-anything
>  Common Ignore
>  Does nothing. Preserved for backward compatibility.
>
> +fasm-show-source
> +Common Var(flag_asm_show_source)
> +Emit comments in the generated assembly code to show the source code
> +lines associated with the assembly instructions.
> +
>  fsanitize=
>  Common Driver Report Joined
>  Select what to sanitize.
> diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
> index 22001f9..dc3d3ad 100644
> --- a/gcc/doc/invoke.texi
> +++ b/gcc/doc/invoke.texi
> @@ -486,7 +486,8 @@ Objective-C and Objective-C++ Dialects}.
>
>  @item Code Generation Options
>  @xref{Code Gen Options,,Options for Code Generation Conventions}.
> -@gccoptlist{-fcall-saved-@var{reg}  -fcall-used-@var{reg} @gol
> +@gccoptlist{-fasm-show-source @gol
> +-fcall-saved-@var{reg}  -fcall-used-@var{reg} @gol
>  -ffixed-@var{reg}  -fexceptions @gol
>  -fnon-call-exceptions  -fdelete-dead-exceptions  -funwind-tables @gol
>  -fasynchronous-unwind-tables @gol
> @@ -11153,6 +11154,74 @@ can figure out the other form by either removing @samp{no-} or adding
>  it.
>
>  @table @gcctabopt
> +@item -fasm-show-source
> +@opindex fasm-show-source
> +Emit comments in the generated assembly code to show the source code
> +lines associated with the assembly instructions.  This option is
> +aimed at end-users who wish to better understand the relationship
> +between their source code and the generated machine code.
> +
> +The comments are of the form FILENAME:LINENUMBER:CONTENT OF LINE.
> +
> +For example, given this C source file:
> +
> +@smallexample
> +int test (int n)
> +@{
> +  int i;
> +  int total = 0;
> +
> +  for (i = 0; i < n; i++)
> +    total += i * i;
> +
> +  return total;
> +@}
> +@end smallexample
> +
> +compiling to (x86_64) assembly via @option{-S} and emitting the result
> +direct to stdout via @option{-o} @option{-}
> +
> +@smallexample
> +gcc -S test.c -fasm-show-source -Os -o -
> +@end smallexample
> +
> +gives output similar to this, highlighting which instructions correspond
> +to the various parts of the loop:
> +
> +@smallexample
> +       .file   "test.c"
> +       .text
> +       .globl  test
> +       .type   test, @@function
> +test:
> +.LFB0:
> +       .cfi_startproc
> +# test.c:4:   int total = 0;
> +       xorl    %eax, %eax
> +# test.c:6:   for (i = 0; i < n; i++)
> +       xorl    %edx, %edx
> +.L2:
> +# test.c:6:   for (i = 0; i < n; i++)
> +       cmpl    %edi, %edx
> +       jge     .L5
> +# test.c:7:     total += i * i;
> +       movl    %edx, %ecx
> +       imull   %edx, %ecx
> +# test.c:6:   for (i = 0; i < n; i++)
> +       incl    %edx
> +# test.c:7:     total += i * i;
> +       addl    %ecx, %eax
> +       jmp     .L2
> +.L5:
> +# test.c:10: @}
> +       ret
> +       .cfi_endproc
> +.LFE0:
> +       .size   test, .-test
> +       .ident  "GCC: (GNU) 7.0.0 20160809 (experimental)"
> +       .section        .note.GNU-stack,"",@@progbits
> +@end smallexample
> +
>  @item -fstack-reuse=@var{reuse-level}
>  @opindex fstack_reuse
>  This option controls stack space reuse for user declared local/auto variables
> diff --git a/gcc/final.c b/gcc/final.c
> index 5b04311..09bf0b7 100644
> --- a/gcc/final.c
> +++ b/gcc/final.c
> @@ -2140,6 +2140,27 @@ call_from_call_insn (rtx_call_insn *insn)
>    return x;
>  }
>
> +/* Implementation of -fasm-show-source.
> +   Print a comment into the asm showing FILENAME, LINENUM, and the
> +   corresponding source line, if available.  */
> +
> +static void
> +asm_show_source (const char *filename, int linenum)
> +{
> +  if (!filename)
> +    return;
> +
> +  int line_size;
> +  const char *line = location_get_source_line (filename, linenum, &line_size);
> +  if (!line)
> +    return;
> +
> +  fprintf (asm_out_file, "%s %s:%i: ", ASM_COMMENT_START, filename, linenum);
> +  /* "line" is not 0-terminated, so we must use line_size.  */
> +  fwrite (line, 1, line_size, asm_out_file);
> +  fputc ('\n', asm_out_file);
> +}
> +
>  /* The final scan for one insn, INSN.
>     Args are same as in `final', except that INSN
>     is the insn being scanned.
> @@ -2563,8 +2584,12 @@ final_scan_insn (rtx_insn *insn, FILE *file, int optimize_p ATTRIBUTE_UNUSED,
>            note in a row.  */
>         if (!DECL_IGNORED_P (current_function_decl)
>             && notice_source_line (insn, &is_stmt))
> -         (*debug_hooks->source_line) (last_linenum, last_filename,
> -                                      last_discriminator, is_stmt);
> +         {
> +           if (flag_asm_show_source)
> +             asm_show_source (last_filename, last_linenum);
> +           (*debug_hooks->source_line) (last_linenum, last_filename,
> +                                        last_discriminator, is_stmt);
> +         }
>
>         if (GET_CODE (body) == PARALLEL
>             && GET_CODE (XVECEXP (body, 0, 0)) == ASM_INPUT)
> diff --git a/gcc/testsuite/gcc.dg/fasm-show-source-1.c b/gcc/testsuite/gcc.dg/fasm-show-source-1.c
> new file mode 100644
> index 0000000..3dd49e0
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/fasm-show-source-1.c
> @@ -0,0 +1,15 @@
> +/* Ensure that the generated asm contains FIXME  */
> +/* { dg-options "-fasm-show-source" } */
> +
> +int test (int n)
> +{
> +  int i;
> +  int total = 0;
> +
> +  for (i = 0; i < n; i++)
> +    total += i * i;
> +
> +  return total;
> +}
> +
> +/* { dg-final { scan-assembler "total = 0" } } */
> --
> 1.8.5.3
>

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