| Issue |
177318
|
| Summary |
[InstCombine] How to eliminate unnecessary the masking instruction in common loops?
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
buggfg
|
When **the IV's initial value is a variable and its end value is a constant**, this causes an unnecessary masking instruction.
```c
void func(int start, int a[100]){
for (int i = start; i < 56; i++){
a[i] += 1;
}
}
```
The corresponding IR is
```c++
define dso_local void @func(i32 noundef %0, ptr noundef captures(none) %1) local_unnamed_addr #0 {
%3 = icmp slt i32 %0, 56
br i1 %3, label %4, label %6
4: ; preds = %2
%5 = sext i32 %0 to i64
br label %7
6: ; preds = %7, %2
ret void
7: ; preds = %4, %7
%8 = phi i64 [ %5, %4 ], [ %12, %7 ]
%9 = getelementptr inbounds i32, ptr %1, i64 %8
%10 = load i32, ptr %9, align 4, !tbaa !5
%11 = add nsw i32 %10, 1
store i32 %11, ptr %9, align 4, !tbaa !5
%12 = add nsw i64 %8, 1
%13 = and i64 %12, 4294967295
%14 = icmp eq i64 %13, 56
br i1 %14, label %6, label %7, !llvm.loop !9
}
```
Currently, I understand that `IndVarSimplifyPass:linearFunctionTestReplace()` generates a trunc instruction when optimizing the loop exit condition, and that `InstCombinePass::foldICmpTruncConstant()` emits this masking instruction when folding "icmp (trunc X), C".
What should be done to reasonably eliminate this unnecessary masking instruction? Thanks!
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