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ARM: fix non-signal-frame local unw_resume() due to compiler optimization cleverness

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When cross-compiling libunwind with optimizations (-O1 or higher),
gcc-4.6.3 (Ubuntu/Linaro 4.6.3-1ubuntu5) optimizes away the memory
writes prior to the inline asm() statement in arm_local_resume() in the
non-signal-frame path, causing the `regs' array to be only allocated on
the stack, but not populated. This means that we are restoring garbage
to the registers.

As suggested in the GCC docs, add a fixed size input memory constraint
for the array content. This is enough to get the desired code to be
generated.

Adding __builtin_unreachable() to the point that we should never reach
was also in itself enough to inhibit the optimization. It also reduces
the function size by a few instructions.
This commit is contained in:
Tommi Rantala 2012-08-10 10:58:57 +03:00
parent 5fef17c05d
commit 14fc15928b
1 changed files with 8 additions and 1 deletions
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9
src/arm/Gresume.c
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@ -1,6 +1,7 @@
/* libunwind - a platform-independent unwind library /* libunwind - a platform-independent unwind library
Copyright (C) 2008 CodeSourcery Copyright (C) 2008 CodeSourcery
Copyright 2011 Linaro Limited Copyright 2011 Linaro Limited
Copyright (C) 2012 Tommi Rantala <tt.rantala@gmail.com>
This file is part of libunwind. This file is part of libunwind.
@ -51,11 +52,16 @@ arm_local_resume (unw_addr_space_t as, unw_cursor_t *cursor, void *arg)
regs[8] = uc->regs[13]; /* SP */ regs[8] = uc->regs[13]; /* SP */
regs[9] = uc->regs[14]; /* LR */ regs[9] = uc->regs[14]; /* LR */
struct regs_overlay {
char x[sizeof(regs)];
};
asm __volatile__ ( asm __volatile__ (
"ldmia %0, {r4-r12, lr}\n" "ldmia %0, {r4-r12, lr}\n"
"mov sp, r12\n" "mov sp, r12\n"
"bx lr\n" "bx lr\n"
: : "r" (regs) : : "r" (regs),
"m" (*(struct regs_overlay *)regs)
); );
} }
else else
@ -90,6 +96,7 @@ arm_local_resume (unw_addr_space_t as, unw_cursor_t *cursor, void *arg)
: : "r" (c->sigcontext_sp), "r" (c->sigcontext_pc) : : "r" (c->sigcontext_sp), "r" (c->sigcontext_pc)
); );
} }
__builtin_unreachable();
#else #else
printf ("%s: implement me\n", __FUNCTION__); printf ("%s: implement me\n", __FUNCTION__);
#endif #endif