https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82158
Bug ID: 82158
Summary: _Noreturn functions that do return clobber caller's
registers on ARM32 (but not other arches)
Product: gcc
Version: 7.1.0
URL: https://godbolt.org/g/GhW4b8
Status: UNCONFIRMED
Keywords: wrong-code
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: peter at cordes dot ca
Target Milestone: ---
Target: arm*-*-*
ARM32 clobbers a call-preserved register in a function which does actually
return. (It was declared _Noreturn, but gcc chooses to make a function which
returns instead of crashing).
Not sure if this is a feature or bug (came up in
https://stackoverflow.com/a/45982153/224132). Probably a bug, since saving
registers in a noreturn function is potentially useful for backtraces (and gcc
avoids tailcall with noreturn for that reason), even if exceptions are disabled
so stack-unwinding doesn't need them. And this is only happening on ARM32, not
the others I looked at: ARM64, MIPS, MIPS64, PowerPC64, MSP430, x86 -m32, or
x86 -m64. (But ARM's multi-operand-at-once POP is probably handled specially,
so that could explain the difference...)
Anyway, in a function declared noreturn which actually *does* return, ARM32
still does the optimization of clobbering "call-preserved" even though it also
generates code to return instead of just falling off the end of the function.
This is C undefined behaviour so it's legal, but is this desirable? gcc warns
about it (even without -Wall), but it's so likely to cause hard-to-debug
problems that I'd suggest erroring by default, or not doing the optimization.
// https://godbolt.org/g/GhW4b8 for gcc6.3, also tested locally with gcc7.1
void ext(void);
ATTRIBUTE
void foo(int *p, int y) {
ext();
*p = y; // then use args that had to survive a call
}
On ARM32 with gcc7.1 -O3 -DATTRIBUTE=_Noreturn produces this:
7 : <source>:7:1: warning: 'noreturn' function does return
foo:
@ Function supports interworking.
@ Volatile: function does not return. # This line not present
without _Noreturn
@ args = 0, pretend = 0, frame = 0
@ frame_needed = 0, uses_anonymous_args = 0
push {r4, lr}
mov r5, r1
mov r4, r0
bl ext
str r5, [r4]
pop {r4, lr}
bx lr
With ATTRIBUTE= (empty), we get push/pop {r4, r5, r6, lr} and things are
otherwise the same. See https://godbolt.org/g/hYnrrD for a diff.
