Summary:
$ clang -O2 -pg -mfentry foo.c
was adding frame pointers to all functions. This was exposed via
compiling the Linux kernel for x86_64 with CONFIG_FUNCTION_TRACER
enabled.
-pg was unconditionally setting the equivalent of -fno-omit-frame-pointer,
regardless of the presence of -mfentry or optimization level. After this
patch, frame pointers will only be omitted at -O0 or if
-fno-omit-frame-pointer is explicitly set for -pg -mfentry.
See also:
https://gcc.gnu.org/git/?p=gcc.git;a=commitdiff;h=3c5273a96ba8dbf98c40bc6d9d0a1587b4cfedb2;hp=c9d75a48c4ea63ab27ccdb40f993236289b243f2#patch2
(modification to ix86_frame_pointer_required())
Fixes: pr/44934
Reviewers: void, manojgupta, dberris, MaskRay, hfinkel
Reviewed By: MaskRay
Subscribers: cfe-commits, llozano, niravd, srhines
Tags: #clang
Differential Revision: https://reviews.llvm.org/D74698
gcc/config/{i386,s390} support -mnop-mcount. We currently only support
-mnop-mcount for SystemZ. The function attribute "mnop-mcount" is
ignored on other targets.
gcc/config/{i386,s390} support -mfentry. We currently only support
-mfentry for X86 and SystemZ. TargetOpcode::FENTRY_CALL is not handled
on other targets.
% clang -target aarch64 -pg -mfentry a.c -c
fatal error: error in backend: Not supported instr: <MCInst 21>
-mfentry, -mrecord-mcount, and -mnop-mcount were invented for Linux
ftrace. Linux uses $(call cc-option-yn,-mrecord-mcount) to detect if the
specific feature is available. Reject unsupported features so that Linux
build system will not wrongly consider them available and cause
build/runtime failures.
Note, GCC has stricter checks that we do not implement, e.g. -fpic/-fpie
-fnop-mcount is not allowed on x86, -fpic/-fpie -mfentry is not allowed
on x86-32.
Nick Desaulniers