[llvm] r344239 - [InstCombine] Demand bits of UMin

[David Green via llvm-commits]

Author: dmgreen
Date: Thu Oct 11 04:28:27 2018
New Revision: 344239

URL: http://llvm.org/viewvc/llvm-project?rev=344239&view=rev
Log:
[InstCombine] Demand bits of UMin

This is the umin alternative to the umax code from rL344237. We use
DeMorgans law on the umax case to bring us to the same thing on umin,
but using countLeadingOnes, not countLeadingZeros.

Differential Revision: https://reviews.llvm.org/D53036

Modified:
    llvm/trunk/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
    llvm/trunk/test/Transforms/InstCombine/minmax-demandbits.ll

Modified: llvm/trunk/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp?rev=344239&r1=344238&r2=344239&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp (original)
+++ llvm/trunk/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp Thu Oct 11 04:28:27 2018
@@ -325,6 +325,16 @@ Value *InstCombiner::SimplifyDemandedUse
       unsigned CTZ = DemandedMask.countTrailingZeros();
       if (match(RHS, m_APInt(C)) && CTZ >= C->getActiveBits())
         return LHS;
+    } else if (SPF == SPF_UMIN) {
+      // UMin(A, C) == A if ...
+      // The lowest non-zero bit of DemandMask is higher than the highest
+      // non-one bit of C.
+      // This comes from using DeMorgans on the above umax example.
+      const APInt *C;
+      unsigned CTZ = DemandedMask.countTrailingZeros();
+      if (match(RHS, m_APInt(C)) &&
+          CTZ >= C->getBitWidth() - C->countLeadingOnes())
+        return LHS;
     }
 
     // If this is a select as part of any other min/max pattern, don't simplify

Modified: llvm/trunk/test/Transforms/InstCombine/minmax-demandbits.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstCombine/minmax-demandbits.ll?rev=344239&r1=344238&r2=344239&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/InstCombine/minmax-demandbits.ll (original)
+++ llvm/trunk/test/Transforms/InstCombine/minmax-demandbits.ll Thu Oct 11 04:28:27 2018
@@ -190,9 +190,7 @@ define i32 @or_umin(i32 %A) {
 
 define i8 @or_min_31_30(i8 %A) {
 ; CHECK-LABEL: @or_min_31_30(
-; CHECK-NEXT:    [[CMP:%.*]] = icmp ult i8 [[A:%.*]], -30
-; CHECK-NEXT:    [[MIN:%.*]] = select i1 [[CMP]], i8 [[A]], i8 -30
-; CHECK-NEXT:    [[R:%.*]] = or i8 [[MIN]], 31
+; CHECK-NEXT:    [[R:%.*]] = or i8 [[A:%.*]], 31
 ; CHECK-NEXT:    ret i8 [[R]]
 ;
   %cmp = icmp ult i8 %A, -30
@@ -203,9 +201,7 @@ define i8 @or_min_31_30(i8 %A) {
 
 define i8 @and_min_7_7(i8 %A) {
 ; CHECK-LABEL: @and_min_7_7(
-; CHECK-NEXT:    [[L2:%.*]] = icmp ult i8 [[A:%.*]], -7
-; CHECK-NEXT:    [[MIN:%.*]] = select i1 [[L2]], i8 [[A]], i8 -7
-; CHECK-NEXT:    [[R:%.*]] = and i8 [[MIN]], -8
+; CHECK-NEXT:    [[R:%.*]] = and i8 [[A:%.*]], -8
 ; CHECK-NEXT:    ret i8 [[R]]
 ;
   %l2 = icmp ult i8 %A, -7
@@ -216,9 +212,7 @@ define i8 @and_min_7_7(i8 %A) {
 
 define i8 @and_min_7_8(i8 %A) {
 ; CHECK-LABEL: @and_min_7_8(
-; CHECK-NEXT:    [[L2:%.*]] = icmp ult i8 [[A:%.*]], -8
-; CHECK-NEXT:    [[MIN:%.*]] = select i1 [[L2]], i8 [[A]], i8 -8
-; CHECK-NEXT:    [[R:%.*]] = and i8 [[MIN]], -8
+; CHECK-NEXT:    [[R:%.*]] = and i8 [[A:%.*]], -8
 ; CHECK-NEXT:    ret i8 [[R]]
 ;
   %l2 = icmp ult i8 %A, -8