| Issue |
117509
|
| Summary |
Const loop not folded
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
ZuseZ4
|
Originally encountered on the Julia Slack, then discussed a bit on the Rust Discord.
The following loop has no side effects and should be simple enough to fold, yet both clang and rustc end up with a phi node.
https://godbolt.org/z/16rG5bjz9
```c++
unsigned long foo() {
unsigned long max = 1000000000;
unsigned long total = 0;
for (unsigned long i = 0; i < max; i++) {
total = i;
}
return total;
}
```
under -O3 and -S -emit-llvm gives
```llvm
define dso_local noundef i64 @foo()() local_unnamed_addr #0 !dbg !10 {
entry:
#dbg_value(i64 1000000000, !16, !DIExpression(), !20)
#dbg_value(i64 0, !17, !DIExpression(), !20)
#dbg_value(i64 0, !18, !DIExpression(), !21)
br label %vector.body, !dbg !22
vector.body:
%index = phi i64 [ 0, %entry ], [ %index.next.9, %vector.body ], !dbg !23
%0 = phi i64 [ 15, %entry ], [ %1, %vector.body ], !dbg !23
%index.next.9 = add nuw nsw i64 %index, 320, !dbg !23
%1 = add nuw nsw i64 %0, 320, !dbg !23
%2 = icmp eq i64 %index.next.9, 1000000000, !dbg !23
br i1 %2, label %for.cond, label %vector.body, !dbg !23
for.cond:
%vector.recur.extract = add nuw i64 %0, 304
#dbg_value(i64 1000000000, !18, !DIExpression(), !21)
#dbg_value(i64 1000000000, !17, !DIExpression(), !20)
#dbg_value(i64 1000000001, !18, !DIExpression(), !21)
ret i64 %vector.recur.extract, !dbg !30
}
attributes #0 = { mustprogress nofree norecurse nosync nounwind willreturn memory(none) uwtable "min-legal-vector-width"="0" "no-trapping-math"="true" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+cmov,+cx8,+fxsr,+mmx,+sse,+sse2,+x87" "tune-cpu"="generic" }
```
The resulting asm is:
```asm
foo():
mov rax, -1
.LBB0_1:
add rax, 320
cmp rax, 999999999
jne .LBB0_1
ret
```
compared to GCC, which just has a
```
foo():
mov eax, 999999999
ret
```
Here the pass that looks suspicious (thanks to @scottamain) https://rust.godbolt.org/z/dG9far1dW
Tbh. it looks like a fun issue, so if someone gives me a hint on where to fix it, I could start trying to come up with a fix next week.
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