Missed fold for switch between two values #51127
Description
| Bugzilla Link | 51785 |
| Version | trunk |
| OS | Windows NT |
| CC | @Kojoley,@rotateright |
Extended Description
LLVM fails to optimize
int foo(int x) {
switch (x) {
case 0: return 0;
case 1: return 1;
}
__builtin_unreachable();
}
into
int foo(int x) {
return x;
}
https://godbolt.org/z/M9YzaWexo
GCC misses this too.
Both LLVM and GCC fold well when another case is added, e.g. case 2: return 2;:
int bar(int x) {
switch (x) {
case 0: return 0;
case 1: return 1;
case 2: return 2;
}
__builtin_unreachable();
}
*** IR Dump After SROA on _Z3fooi ***
; Function Attrs: mustprogress nounwind uwtable
define dso_local i32 @_Z3fooi(i32 %0) #0 {
%2 = icmp ult i32 %0, 1
br i1 %2, label %3, label %4
3: ; preds = %1
br label %5
4: ; preds = %1
br label %5
5: ; preds = %4, %3
%6 = phi i32 [ 0, %3 ], [ 1, %4 ]
ret i32 %6
}
*** IR Dump After SimplifyCFGPass on _Z3fooi ***
; Function Attrs: mustprogress nounwind uwtable
define dso_local i32 @_Z3fooi(i32 %0) local_unnamed_addr #0 {
%2 = icmp eq i32 %0, 0
%3 = select i1 %2, i32 0, i32 1
ret i32 %3
}
After the SimplifyCFGPass here, the information required regarding undefined paths to perform the fold on foo is lost and the transformation cannot be made correctly later.
For bar, the optimization is actually done in the SimplifyCFGPass:
*** IR Dump After InstCombinePass on _Z3fooi ***
; Function Attrs: mustprogress nofree norecurse nosync nounwind readnone uwtable willreturn
define dso_local i32 @_Z3fooi(i32 %0) local_unnamed_addr #0 {
switch i32 %0, label %4 [
i32 0, label %5
i32 1, label %2
i32 2, label %3
]
2: ; preds = %1
br label %5
3: ; preds = %1
br label %5
4: ; preds = %1
unreachable
5: ; preds = %1, %3, %2
%6 = phi i32 [ 2, %3 ], [ 1, %2 ], [ 0, %1 ]
ret i32 %6
}
*** IR Dump After SimplifyCFGPass on _Z3fooi ***
; Function Attrs: mustprogress nofree norecurse nosync nounwind readnone uwtable willreturn
define dso_local i32 @_Z3fooi(i32 %0) local_unnamed_addr #0 {
ret i32 %0
}