https://bugs.llvm.org/show_bug.cgi?id=50171
Bug ID: 50171
Summary: Missed optimization to remove unnecessary branch from
loop entry
Product: clang
Version: unspecified
Hardware: PC
OS: Windows NT
Status: NEW
Severity: enhancement
Priority: P
Component: C++
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected], [email protected],
[email protected], [email protected],
[email protected]
The following toy example:
void Loop(int len) {
int i = 0;
const int kUnrollFactor = 8;
for (int num_calls = 0; i <= len - kUnrollFactor; ) {
if (num_calls + kUnrollFactor > 100) {
extern void Foo(); Foo();
num_calls = 0;
}
for (int j = 0; j < kUnrollFactor; j++, i++, num_calls++) {
extern void Bar(int); Bar(i);
}
}
}
Compiles to the following x86 assembly code with clang-9:
Loop(int):
... prolog ...
mov r14d, edi
add r14d, -8
js .LBB0_5
xor r15d, r15d <=== num_calls = 0
xor ebx, ebx
cmp r15d, 93 <=== num_calls still zero here
jge .LBB0_3 <=== branch can NEVER be taken
.LBB0_4:
... unrolled loop body with 8 calls to Bar() ...
add r15d, 8
add ebx, 8
cmp ebp, r14d
jge .LBB0_5
cmp r15d, 93
jl .LBB0_4
.LBB0_3:
call Foo()
xor r15d, r15d
jmp .LBB0_4
.LBB0_5:
... epilogue ...
ret
Ideally, the compiler should elide the provably redundant cmp+jge pair, leaving
only the xor:
xor r15d, r15d
cmp r15d, 93
jge .LBB0_3
With clang-12, the result is arguably worse, because the extra branching masks
the missed opportunity altogether:
Loop(int):
... prologue ...
cmp edi, 8
jge .LBB0_1
.LBB0_5:
... epilogue ...
ret
.LBB0_1:
... loop initialization ...
xor r15d, r15d <=== num_calls = 0
xor ebx, ebx
jmp .LBB0_2
.LBB0_4:
... unrolled loop body with 8 calls to Bar() ...
add r15d, 8
add ebx, 8
cmp ebp, r14d
jge .LBB0_5
.LBB0_2:
cmp r15d, 93 <=== num_calls = 0 the first time
jl .LBB0_4 <=== branch never taken the first time
call Foo()
xor r15d, r15d
jmp .LBB0_4
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