https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109534
Bug ID: 109534
Summary: -fdirectives-only does not work with
assembler-with-cpp
Product: gcc
Version: 12.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: boris at kolpackov dot net
Target Milestone: ---
I've noticed that -fdirectives-only has no effect when preprocessing
assembler-with-cpp files. Instead, one always get a fully preprocessed source:
$ cat <<EOF >test.S
#define MAIN_RESULT 2
.text
.global main
main:
movq $MAIN_RESULT, %rax
ret
EOF
$ gcc -E -fdirectives-only -o test.Si test.S
$ cat test.Si
# 0 "test.S"
# 0 "<built-in>"
# 0 "<command-line>"
# 1 "/usr/include/stdc-predef.h" 1 3 4
# 0 "<command-line>" 2
# 1 "test.S"
.text
.global main
main:
movq $2, %rax
ret
Running the same command with -v reveals that -fdirectives-only is overriden
with -fno-directives-only:
$ gcc -v -E -fdirectives-only -o test.Si test.S
...
/usr/lib/gcc/x86_64-linux-gnu/12/cc1 -E -lang-asm -quiet -v -imultiarch
x86_64-linux-gnu test.S -o test.Si -mtune=generic -march=x86-64
-fdirectives-only -fasynchronous-unwind-tables -fno-directives-only -dumpbase
test.S -dumpbase-ext .S
...
And I've tracked this down to the following old gcc/ChangeLog-2007 entry:
2007-08-30 Ollie Wild <a...@google.com>
* c-opts.c (c_common_handle_option): Support -fno-directives-only.
* gcc.c (default_compilers): Add -fno-directives-only to
@assembler-with-cpp.
Unfortunately there is no rationale (I've also checked the commit message and
there is nothing there either).
I then tried to manually remove -fno-directives-only and see what happens:
$ /usr/lib/gcc/x86_64-linux-gnu/12/cc1 -E -lang-asm -quiet -v -imultiarch
x86_64-linux-gnu test.S -o test.Si -mtune=generic -march=x86-64
-fdirectives-only -fasynchronous-unwind-tables -dumpbase test.S -dumpbase-ext
.S
The resulting test.Si file looked sensible so I tried to compile it further:
$ gcc -v -x assembler-with-cpp -fpreprocessed -fdirectives-only main.Sii
But -fno-directives-only is passed in this case as well so I had to tweak and
run the underlying commands manually:
$ /usr/lib/gcc/x86_64-linux-gnu/12/cc1 -E -lang-asm -quiet -v -imultiarch
x86_64-linux-gnu test.Si -mtune=generic -march=x86-64 -fpreprocessed
-fdirectives-only -fasynchronous-unwind-tables -o test.s
$ as -v --64 -o test.o test.s
test.Si:1: Error: junk at end of line, first unrecognized character is `0'
The beginning of test.s looks like this:
$ head -4 test.s
# 1 "test.Si"
0 "test.S"
# 0 "<built-in>"
# 0 "<built-in>"
The second line definitely looks wrong (missing #? maybe that's what the above
commit was hacking around?). But if I fix that, for example, by adding the
leading #, the rest looks correct and everything compiles, links, and runs as
expected.
So it seems like making -fdirectives-only work for assembler-with-cpp is a
matter of tracking down and fixing that broken line directive generation.
