Teaching SimplifyCFG to recognize the Or-Mask trick that InstCombine uses to

reduce the number of comparisons.

Specifically, InstCombine can turn:
  (i == 5334 || i == 5335)
into:
  ((i | 1) == 5335)

SimplifyCFG was already able to detect the pattern:
  (i == 5334 || i == 5335)
to:
  ((i & -2) == 5334)

This patch supersedes D21315 and resolves PR27555
(https://llvm.org/bugs/show_bug.cgi?id=27555).

Thanks to David and Chandler for the suggestions!

Author: Thomas Jablin (tjablin)
Reviewers: majnemer chandlerc halfdan cycheng

http://reviews.llvm.org/D21397

llvm-svn: 273639

1 files changed, 39 insertions, 0 deletions

diff --git a/llvm/test/Transforms/SimplifyCFG/switch_create.ll b/llvm/test/Transforms/SimplifyCFG/switch_create.ll
index 75498faaa29..29d3a34a05e 100644
--- a/llvm/test/Transforms/SimplifyCFG/switch_create.ll
+++ b/llvm/test/Transforms/SimplifyCFG/switch_create.ll
@@ -618,4 +618,43 @@ if:
 
 else:
   ret void
+}
+
+; Form a switch when or'ing a power of two
+; CHECK-LABEL: define void @test21
+; CHECK: i32 32, label %else
+; CHECK: i32 13, label %else
+; CHECK: i32 12, label %else
+define void @test21(i32 %arg) {
+  %and = or i32 %arg, 1
+  %cmp1 = icmp ne i32 %and, 13
+  %cmp2 = icmp ne i32 %arg, 32
+  %pred = and i1 %cmp1, %cmp2
+  br i1 %pred, label %if, label %else
+
+if:
+  call void @foo1()
+  ret void
+
+else:
+  ret void
+}
+
+; Since %cmp1 is always false, a switch is never formed
+; CHECK-LABEL: define void @test22
+; CHECK-NOT: switch
+; CHECK: ret void
+define void @test22(i32 %arg) {
+  %and = or i32 %arg, 1
+  %cmp1 = icmp ne i32 %and, 12
+  %cmp2 = icmp ne i32 %arg, 32
+  %pred = and i1 %cmp1, %cmp2
+  br i1 %pred, label %if, label %else
+
+if:
+  call void @foo1()
+  ret void
+
+else:
+  ret void
 }
\ No newline at end of file