On 08/12/2016 01:13 PM, David Malcolm wrote:
On Thu, 2016-08-11 at 20:00 -0600, Sandra Loosemore wrote:
On 08/11/2016 02:34 PM, David Malcolm 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.
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®rtested on x86_64-pc-linux-gnu; adds
2 PASS results to gcc.sum.
Thoughts? OK for trunk as-is?
Why not extend the existing -fverbose-asm to do this? E.g.
-fverbose-asm=source, or something like that. Otherwise you need to
cross-reference the documentation for the two options and explain how
they interact (or don't interact, as the case may be).
-Sandra
Thanks.
With the patch as-is, if I pass both -fverbose-asm and
-fasm-show-source, I get the following:
# test.c:7: int total = 0;
xorl %eax, %eax # <retval>
# test.c:9: for (i = 0; i < n; i++)
xorl %edx, %edx # i
.L2:
# test.c:9: for (i = 0; i < n; i++)
cmpl %edi, %edx # n, i
jge .L5 #,
# test.c:10: total += i * i;
movl %edx, %ecx # i, tmp92
imull %edx, %ecx # i, tmp92
# test.c:9: for (i = 0; i < n; i++)
incl %edx # i
# test.c:10: total += i * i;
addl %ecx, %eax # tmp92, <retval>
jmp .L2 #
.L5:
# test.c:13: }
ret
.cfi_endproc
I find the above pleasing, as it shows both the source, and the
variable names associated with the asm insn arguments. The source
line information works well with jump-to-source, in Emacs, at least.
-fverbose-asm also adds some metadata to the top of the dump, that I
wasn't interested in from a see-the-source point of view, but I can live
with that.
Currently -fverbose-asm is documented in common.opt as:
"Add extra commentary to assembler output"
and in invoke.texi as:
@opindex fverbose-asm
Put extra commentary information in the generated assembly code to
make it more readable. This option is generally only of use to those
who actually need to read the generated assembly code (perhaps while
debugging the compiler itself).
@option{-fno-verbose-asm}, the default, causes the
extra information to be omitted and is useful when comparing two
assembler files.
Given that the precise output format for -fverbose-asm isn't
documented, and it already can be thought of as our option for
"make the asm readable please", my preference would be to extend it
to print the source lines, without adding any "=source"
complications: if the user was interested in the relationship of ins
arguments to source expressions, they're likely also interested in
seeing the corresponding source lines.
Following is a rewritten version of the patch that does this, adding
a description of the output to invoke.texi (along with a caveat that
one should not try to parse the comments).
OK for trunk if it survives bootstrap®ression testing?
As mentioned before, I'd also like to make this more discoverable,
by automatically adding the option in the driver if the user has
specified asm output (via -S or via "-o something.s") - is this
latter idea also reasonable?
Dave
gcc/ChangeLog:
* doc/invoke.texi (fverbose-asm): Note that source code lines
are emitted, and provide an example.
* final.c (asm_show_source): New function.
(final_scan_insn): Call asm_show_source.
gcc/testsuite/ChangeLog:
* gcc.dg/verbose-asm-2.c: New test case.
I don't think we have any kind of requirement to maintain a format for
-fverbose-asm. So I think adding the new stuff under the -fverbose-asm
flag is the way to go.
Whether or not to add it to -S or not is a distinct question and I don't
think the answer is as clear cut...
OK on the updated patch.
jeff
