[InstCombine] canonicalize min/max constant to select's false value

This is a first step towards canonicalization and improved folding/codegen
for integer min/max as discussed here:
http://lists.llvm.org/pipermail/llvm-dev/2016-November/106868.html

Here, we're just matching the simplest min/max patterns and adjusting the
icmp predicate while swapping the select operands.

I've included FIXME tests in test/Transforms/InstCombine/select_meta.ll
so it's easier to see how this might be extended (corresponds to the TODO
comment in the code). That's also why I'm using matchSelectPattern()
rather than a simpler check; once the backend is patched, we can just 
remove some of the restrictions to allow the obfuscated min/max patterns
in the FIXME tests to be matched.

Differential Revision: https://reviews.llvm.org/D26525

llvm-svn: 287585

1 files changed, 2 insertions, 2 deletions

diff --git a/llvm/test/Transforms/InstCombine/div-shift.ll b/llvm/test/Transforms/InstCombine/div-shift.ll
index 053210650d1..517313ed8e4 100644
--- a/llvm/test/Transforms/InstCombine/div-shift.ll
+++ b/llvm/test/Transforms/InstCombine/div-shift.ll
@@ -45,8 +45,8 @@ define i64 @t3(i64 %x, i32 %y) {
 
 define i32 @t4(i32 %x, i32 %y) {
 ; CHECK-LABEL: @t4(
-; CHECK-NEXT:    [[TMP1:%.*]] = icmp ult i32 %y, 5
-; CHECK-NEXT:    [[DOTV:%.*]] = select i1 [[TMP1]], i32 5, i32 %y
+; CHECK-NEXT:    [[TMP1:%.*]] = icmp ugt i32 %y, 5
+; CHECK-NEXT:    [[DOTV:%.*]] = select i1 [[TMP1]], i32 %y, i32 5
 ; CHECK-NEXT:    [[TMP2:%.*]] = lshr i32 %x, [[DOTV]]
 ; CHECK-NEXT:    ret i32 [[TMP2]]
 ;