https://bugs.llvm.org/show_bug.cgi?id=37535
Bug ID: 37535
Summary: Dead condition left after removal of branch that used
it
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Keywords: performance
Severity: enhancement
Priority: P
Component: Backend: ARM
Assignee: unassignedb...@nondot.org
Reporter: lidija.bes...@rt-rk.com
CC: llvm-bugs@lists.llvm.org, mips3...@gmail.com
I have been looking into some code compiled for ARM32 using LLVM and I noticed
that in some cases -O3 and -Os optimizations leave excess instructions. I have
made a mini example where that can be seen:
typedef union df_union {
int *set;
int state;
} df;
int n;
df *d;
int foo1(int index){
if ((n && !d[index].set) || (!n && !d[index].state)) {
return --index;
}
return 0;
}
For this clang generated:
00000000 <foo1>:
0: e59f1024 ldr r1, [pc, #36] ; 2c <foo1+0x2c>
4: e59f2024 ldr r2, [pc, #36] ; 30 <foo1+0x30>
8: e5922000 ldr r2, [r2]
c: e5911000 ldr r1, [r1]
10: e3520000 cmp r2, #0
14: e7912100 ldr r2, [r1, r0, lsl #2]
18: e3a01000 mov r1, #0
1c: e3520000 cmp r2, #0
20: 02401001 subeq r1, r0, #1
24: e1a00001 mov r0, r1
28: e12fff1e bx lr
cmp and loads that it uses are excess and when removed function continues to
give expected result.
I had a look into it and it appears to be caused by merging of basic blocks and
removal of conditional branch whose condition is computed using excess
instructions which are left behind. This happens in Control Flow Optimizes pass
in BranchFolding.cpp in function OptimizeBlock where ARM's removeBranch is
called, none of them removes condition calculation.
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