[llvm-bugs] [Bug 51785] New: Missed fold for switch between two values

[via llvm-bugs]

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: unassignedbugs at nondot.org
          Reporter: llvm at rifkin.dev
                CC: llvm-bugs at lists.llvm.org

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
}

-- 
You are receiving this mail because:
You are on the CC list for the bug.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-bugs/attachments/20210907/77e9923b/attachment.html>