https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109903
Bug ID: 109903
Summary: Register misallocation in hand-crafted asm insn, no
diagnostics produced
Product: gcc
Version: 14.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: dimitri.gorokhovik at free dot fr
Target Milestone: ---
For the source code:
#if NO_BUG
unsigned long f (unsigned long a)
{
unsigned long v;
asm ("mrs %[_1], pmccntr_eli0\n"
"\tldr x1, [%[_2]]\n"
: [_1] "=r" (v)
: [_2] "r" (a)
);
return v;
}
#else // BUG
extern void g(unsigned long);
void f (unsigned long a)
{
unsigned long v;
asm ("mrs %[_1], pmccntr_eli0\n"
"\tldr x1, [%[_2]]\n"
: [_1] "=r" (v)
: [_2] "r" (a)
);
g(v);
}
#endif
The asm output produced is:
## BUG
#APP
# 23 "bug.c" 1
mrs %rdi, pmccntr_eli0
ldr x1, [%rdi]
# 0 "" 2
#NO_APP
## NO_BUG
#APP
# 6 "bug.c" 1
mrs %rax, pmccntr_eli0
ldr x1, [%rdi]
# 0 "" 2
#NO_APP
As one can see, in the "NO BUG" case, the registers allocated to reading "the
counter" and to reading "memory", are different, whereas they are one and the
same in the "BUG" case.
This was obtained with today's trunk built for native compilation: gcc (GCC)
14.0.0 20230518 (experimental).
The example was compiled as:
gcc -S -O3 -Wall -Wextra -o bug.S bug.c
no diagnostic messages are produced.
Same was also observed with gcc-aarch64-none-linux toolchain 13.2 (from Arm
download page), so "target-related issue" might not be an entirely correct
classification for this.
