https://bugs.llvm.org/show_bug.cgi?id=46039
Bug ID: 46039
Summary: opt -simplifycfg -lowerswitch makes code worse
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: Transformation Utilities
Assignee: unassignedb...@nondot.org
Reporter: jay.f...@gmail.com
CC: llvm-bugs@lists.llvm.org
This example starts out as "if (x==1) ... else if (x==2) ...". First
-simplifycfg converts the ifs to a switch. Then -lowerswitch converts the
switch back to ifs, but it comes up with worse code than we started with: "if
(x<2) { if (x==1) ... } else { if (x==2) ... }". This seems like a missed
optimization in -lowerswitch.
$ cat a.ll
define void @f(i32 %x, i32* %p) {
bb0:
%c1 = icmp eq i32 %x, 1
br i1 %c1, label %bb1, label %bb2
bb1:
store i32 33, i32* %p
br label %bb4
bb2:
%c2 = icmp eq i32 %x, 2
br i1 %c2, label %bb3, label %bb4
bb3:
store i32 66, i32* %p
br label %bb4
bb4:
ret void
}
$ opt -simplifycfg -lowerswitch a.ll -S -o -
; ModuleID = 'a.ll'
source_filename = "a.ll"
define void @f(i32 %x, i32* %p) {
bb0:
br label %NodeBlock
NodeBlock: ; preds = %bb0
%Pivot = icmp slt i32 %x, 2
br i1 %Pivot, label %LeafBlock, label %LeafBlock1
LeafBlock1: ; preds = %NodeBlock
%SwitchLeaf2 = icmp eq i32 %x, 2
br i1 %SwitchLeaf2, label %bb3, label %NewDefault
LeafBlock: ; preds = %NodeBlock
%SwitchLeaf = icmp eq i32 %x, 1
br i1 %SwitchLeaf, label %bb1, label %NewDefault
bb1: ; preds = %LeafBlock
store i32 33, i32* %p, align 4
br label %bb4
bb3: ; preds = %LeafBlock1
store i32 66, i32* %p, align 4
br label %bb4
NewDefault: ; preds = %LeafBlock1,
%LeafBlock
br label %bb4
bb4: ; preds = %NewDefault, %bb3,
%bb1
ret void
}
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