https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109343
Bug ID: 109343
Summary: invalid if conversion optimization for aarch64
Product: gcc
Version: rust/master
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: rtl-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: jiangning.liu at amperecomputing dot com
Target Milestone: ---
For this small case, if-conversion optimization in back-end generated csel
instruction for aarch64, which is unsafe. The address of variable sga_var could
be invalid if sga_mapped is false.
$ cat ttt2.c
extern int sga_mapped, sga_var;
int func(int j){
int i=0;
if(sga_mapped)
i=i+sga_var;
return i;
}
$ gcc -O3 -S ttt2.c
$ cat ttt2.s
.arch armv8-a
.file "ttt2.c"
.text
.align 2
.p2align 4,,11
.global func
.type func, %function
func:
.LFB0:
.cfi_startproc
adrp x0, sga_mapped
adrp x1, sga_var
ldr w0, [x0, #:lo12:sga_mapped]
ldr w1, [x1, #:lo12:sga_var]
cmp w0, 0
csel w0, w1, w0, ne
ret
.cfi_endproc
.LFE0:
.size func, .-func
.ident "GCC: (GNU) 12.2.1 20221121 (Red Hat 12.2.1-4)"
.section .note.GNU-stack,"",@progbits
For x86, the following code is generated. It is safe because the memory access
to sga_var(%rip) won't be really triggered if %eax is not set. Here x86 and
aarch64 are different.
$ cat ttt2.s
.file "ttt2.c"
.text
.p2align 4
.globl func
.type func, @function
func:
.LFB0:
.cfi_startproc
endbr64
movl sga_mapped(%rip), %eax
testl %eax, %eax
cmovne sga_var(%rip), %eax
ret
