https://bugs.llvm.org/show_bug.cgi?id=51785
Bug ID: 51785
Summary: Missed fold for switch between two values
Product: libraries
Version: trunk
Hardware: PC
OS: Windows NT
Status: NEW
Severity: enhancement
Priority: P
Component: Scalar Optimizations
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
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
}
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