| Issue |
76705
|
| Summary |
[LVI] Suboptimal evaluation order
|
| Labels |
llvm:optimizations,
missed-optimization
|
| Assignees |
|
| Reporter |
nikic
|
Consider this example:
```llvm
define i1 @test(i32 %x) {
%a = add nuw nsw i32 %x, 1
%b = add nuw nsw i32 %a, 1
%c = add nuw nsw i32 %a, %b
%d = icmp ugt i32 %c, 2
ret i1 %d
}
```
The comparison can be folded to true, but `-passes=correlated-propagation` fails to do so. The relevant LVI log looks like this:
```
LVI Getting block end value %c = add nuw nsw i32 %a, %b at ''
PUSH: %c = add nuw nsw i32 %a, %b in
PUSH: %a = add nuw nsw i32 %x, 1 in
PUSH: %b = add nuw nsw i32 %a, 1 in
POP %b = add nuw nsw i32 %a, 1 in = constantrange<1, 0>
PUSH: i32 %x in
POP i32 %x in = overdefined
POP %a = add nuw nsw i32 %x, 1 in = constantrange<1, 0>
POP %c = add nuw nsw i32 %a, %b in = constantrange<2, 0>
Result = constantrange<2, 0>
```
Notably, the values get pushed in the order `%c, %a, %b`, which means that `%b` gets evaluated before `%a`. But `%b` depends on `%a`, so it will use an overdefined value, rather than the more precise range we later compute.
We can get it to fold by slightly varying the input like this:
```llvm
define i1 @test(i32 %x) {
%a = add nuw nsw i32 %x, 1
%b = add nuw i32 %a, 1
%c = add nuw nsw i32 %a, %b
%d = icmp ugt i32 %c, 2
ret i1 %d
}
```
Removing the nsw flag makes CVP issue a query for `%a` earlier, avoiding the problem.
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