[ValueTracking] recognize variations of 'clamp' to improve codegen (PR31693)

By enhancing value tracking, we allow an existing min/max canonicalization to
kick in and improve codegen for several targets that have min/max instructions.

Unfortunately, recognizing min/max in value tracking may cause us to hit
a hack in InstCombiner::visitICmpInst() more often:
http://lists.llvm.org/pipermail/llvm-dev/2017-January/109340.html
...but I'm hoping we can remove that soon.

Correctness proofs based on Alive:

Name: smaxmin
Pre: C1 < C2
%cmp2 = icmp slt i8 %x, C2
%min = select i1 %cmp2, i8 %x, i8 C2
%cmp3 = icmp slt i8 %x, C1
%r = select i1 %cmp3, i8 C1, i8 %min
=>
%cmp2 = icmp slt i8 %x, C2
%min = select i1 %cmp2, i8 %x, i8 C2
%cmp1 = icmp sgt i8 %min, C1
%r = select i1 %cmp1, i8 %min, i8 C1

Name: sminmax
Pre: C1 > C2
%cmp2 = icmp sgt i8 %x, C2
%max = select i1 %cmp2, i8 %x, i8 C2
%cmp3 = icmp sgt i8 %x, C1
%r = select i1 %cmp3, i8 C1, i8 %max
=>
%cmp2 = icmp sgt i8 %x, C2
%max = select i1 %cmp2, i8 %x, i8 C2
%cmp1 = icmp slt i8 %max, C1
%r = select i1 %cmp1, i8 %max, i8 C1

---------------------------------------- 
Optimization: smaxmin 
Done: 1 
Optimization is correct! 
---------------------------------------- 
Optimization: sminmax 
Done: 1 
Optimization is correct!



Name: umaxmin
Pre: C1 u< C2
%cmp2 = icmp ult i8 %x, C2
%min = select i1 %cmp2, i8 %x, i8 C2
%cmp3 = icmp ult i8 %x, C1
%r = select i1 %cmp3, i8 C1, i8 %min
=>
%cmp2 = icmp ult i8 %x, C2
%min = select i1 %cmp2, i8 %x, i8 C2
%cmp1 = icmp ugt i8 %min, C1
%r = select i1 %cmp1, i8 %min, i8 C1

Name: uminmax
Pre: C1 u> C2
%cmp2 = icmp ugt i8 %x, C2
%max = select i1 %cmp2, i8 %x, i8 C2
%cmp3 = icmp ugt i8 %x, C1
%r = select i1 %cmp3, i8 C1, i8 %max
=>
%cmp2 = icmp ugt i8 %x, C2
%max = select i1 %cmp2, i8 %x, i8 C2
%cmp1 = icmp ult i8 %max, C1
%r = select i1 %cmp1, i8 %max, i8 C1

---------------------------------------- 
Optimization: umaxmin 
Done: 1 
Optimization is correct! 
---------------------------------------- 
Optimization: uminmax 
Done: 1 
Optimization is correct! 
 

llvm-svn: 292660

1 files changed, 8 insertions, 8 deletions

diff --git a/llvm/test/Transforms/InstCombine/minmax-fold.ll b/llvm/test/Transforms/InstCombine/minmax-fold.ll
index ec838e25f31..adb99c283c2 100644
--- a/llvm/test/Transforms/InstCombine/minmax-fold.ll
+++ b/llvm/test/Transforms/InstCombine/minmax-fold.ll
@@ -348,8 +348,8 @@ define i32 @clamp_signed1(i32 %x) {
 ; CHECK-LABEL: @clamp_signed1(
 ; CHECK-NEXT:    [[CMP2:%.*]] = icmp slt i32 %x, 255
 ; CHECK-NEXT:    [[MIN:%.*]] = select i1 [[CMP2]], i32 %x, i32 255
-; CHECK-NEXT:    [[CMP1:%.*]] = icmp slt i32 %x, 15
-; CHECK-NEXT:    [[R:%.*]] = select i1 [[CMP1]], i32 15, i32 [[MIN]]
+; CHECK-NEXT:    [[TMP1:%.*]] = icmp sgt i32 [[MIN]], 15
+; CHECK-NEXT:    [[R:%.*]] = select i1 [[TMP1]], i32 [[MIN]], i32 15
 ; CHECK-NEXT:    ret i32 [[R]]
 ;
   %cmp2 = icmp slt i32 %x, 255
@@ -365,8 +365,8 @@ define i32 @clamp_signed2(i32 %x) {
 ; CHECK-LABEL: @clamp_signed2(
 ; CHECK-NEXT:    [[CMP2:%.*]] = icmp sgt i32 %x, 15
 ; CHECK-NEXT:    [[MAX:%.*]] = select i1 [[CMP2]], i32 %x, i32 15
-; CHECK-NEXT:    [[CMP1:%.*]] = icmp sgt i32 %x, 255
-; CHECK-NEXT:    [[R:%.*]] = select i1 [[CMP1]], i32 255, i32 [[MAX]]
+; CHECK-NEXT:    [[TMP1:%.*]] = icmp slt i32 [[MAX]], 255
+; CHECK-NEXT:    [[R:%.*]] = select i1 [[TMP1]], i32 [[MAX]], i32 255
 ; CHECK-NEXT:    ret i32 [[R]]
 ;
   %cmp2 = icmp sgt i32 %x, 15
@@ -382,8 +382,8 @@ define i32 @clamp_unsigned1(i32 %x) {
 ; CHECK-LABEL: @clamp_unsigned1(
 ; CHECK-NEXT:    [[CMP2:%.*]] = icmp ult i32 %x, 255
 ; CHECK-NEXT:    [[MIN:%.*]] = select i1 [[CMP2]], i32 %x, i32 255
-; CHECK-NEXT:    [[CMP1:%.*]] = icmp ult i32 %x, 15
-; CHECK-NEXT:    [[R:%.*]] = select i1 [[CMP1]], i32 15, i32 [[MIN]]
+; CHECK-NEXT:    [[TMP1:%.*]] = icmp ugt i32 [[MIN]], 15
+; CHECK-NEXT:    [[R:%.*]] = select i1 [[TMP1]], i32 [[MIN]], i32 15
 ; CHECK-NEXT:    ret i32 [[R]]
 ;
   %cmp2 = icmp ult i32 %x, 255
@@ -399,8 +399,8 @@ define i32 @clamp_unsigned2(i32 %x) {
 ; CHECK-LABEL: @clamp_unsigned2(
 ; CHECK-NEXT:    [[CMP2:%.*]] = icmp ugt i32 %x, 15
 ; CHECK-NEXT:    [[MAX:%.*]] = select i1 [[CMP2]], i32 %x, i32 15
-; CHECK-NEXT:    [[CMP1:%.*]] = icmp ugt i32 %x, 255
-; CHECK-NEXT:    [[R:%.*]] = select i1 [[CMP1]], i32 255, i32 [[MAX]]
+; CHECK-NEXT:    [[TMP1:%.*]] = icmp ult i32 [[MAX]], 255
+; CHECK-NEXT:    [[R:%.*]] = select i1 [[TMP1]], i32 [[MAX]], i32 255
 ; CHECK-NEXT:    ret i32 [[R]]
 ;
   %cmp2 = icmp ugt i32 %x, 15