[llvm] r335622 - [InstCombine] fold urem with sext bool divisor

[Sanjay Patel via llvm-commits]

Author: spatel
Date: Tue Jun 26 09:30:00 2018
New Revision: 335622

URL: http://llvm.org/viewvc/llvm-project?rev=335622&view=rev
Log:
[InstCombine] fold urem with sext bool divisor

Similar to other patches in this series:
https://reviews.llvm.org/rL335512
https://reviews.llvm.org/rL335527
https://reviews.llvm.org/rL335597
https://reviews.llvm.org/rL335616

...this is filling a gap in analysis that is exposed by an unrelated select-of-constants transform.
I didn't see a way to unify the sext cases because each div/rem opcode results in a different fold.

Note that in this case, the backend might want to convert the select into math:
Name: sext urem
%e = sext i1 %x to i32
%r = urem i32 %y, %e
=>
%c = icmp eq i32 %y, -1
%z = zext i1 %c to i32
%r = add i32 %z, %y

Modified:
    llvm/trunk/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
    llvm/trunk/test/Transforms/InstCombine/rem.ll

Modified: llvm/trunk/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp?rev=335622&r1=335621&r2=335622&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp (original)
+++ llvm/trunk/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp Tue Jun 26 09:30:00 2018
@@ -1295,8 +1295,9 @@ Instruction *InstCombiner::visitURem(Bin
 
   // X urem Y -> X and Y-1, where Y is a power of 2,
   Value *Op0 = I.getOperand(0), *Op1 = I.getOperand(1);
+  Type *Ty = I.getType();
   if (isKnownToBeAPowerOfTwo(Op1, /*OrZero*/ true, 0, &I)) {
-    Constant *N1 = Constant::getAllOnesValue(I.getType());
+    Constant *N1 = Constant::getAllOnesValue(Ty);
     Value *Add = Builder.CreateAdd(Op1, N1);
     return BinaryOperator::CreateAnd(Op0, Add);
   }
@@ -1304,7 +1305,7 @@ Instruction *InstCombiner::visitURem(Bin
   // 1 urem X -> zext(X != 1)
   if (match(Op0, m_One())) {
     Value *Cmp = Builder.CreateICmpNE(Op1, Op0);
-    Value *Ext = Builder.CreateZExt(Cmp, I.getType());
+    Value *Ext = Builder.CreateZExt(Cmp, Ty);
     return replaceInstUsesWith(I, Ext);
   }
 
@@ -1315,6 +1316,16 @@ Instruction *InstCombiner::visitURem(Bin
     return SelectInst::Create(Cmp, Op0, Sub);
   }
 
+  // If the divisor is a sext of a boolean, then the divisor must be max
+  // unsigned value (-1). Therefore, the remainder is Op0 unless Op0 is also
+  // max unsigned value. In that case, the remainder is 0:
+  // urem Op0, (sext i1 X) --> (Op0 == -1) ? 0 : Op0
+  Value *X;
+  if (match(Op1, m_SExt(m_Value(X))) && X->getType()->isIntOrIntVectorTy(1)) {
+    Value *Cmp = Builder.CreateICmpEQ(Op0, ConstantInt::getAllOnesValue(Ty));
+    return SelectInst::Create(Cmp, ConstantInt::getNullValue(Ty), Op0);
+  }
+
   return nullptr;
 }
 

Modified: llvm/trunk/test/Transforms/InstCombine/rem.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstCombine/rem.ll?rev=335622&r1=335621&r2=335622&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/InstCombine/rem.ll (original)
+++ llvm/trunk/test/Transforms/InstCombine/rem.ll Tue Jun 26 09:30:00 2018
@@ -60,8 +60,8 @@ define i5 @biggest_divisor(i5 %x) {
 
 define i8 @urem_with_sext_bool_divisor(i1 %x, i8 %y) {
 ; CHECK-LABEL: @urem_with_sext_bool_divisor(
-; CHECK-NEXT:    [[S:%.*]] = sext i1 [[X:%.*]] to i8
-; CHECK-NEXT:    [[REM:%.*]] = urem i8 [[Y:%.*]], [[S]]
+; CHECK-NEXT:    [[TMP1:%.*]] = icmp eq i8 [[Y:%.*]], -1
+; CHECK-NEXT:    [[REM:%.*]] = select i1 [[TMP1]], i8 0, i8 [[Y]]
 ; CHECK-NEXT:    ret i8 [[REM]]
 ;
   %s = sext i1 %x to i8
@@ -71,8 +71,8 @@ define i8 @urem_with_sext_bool_divisor(i
 
 define <2 x i8> @urem_with_sext_bool_divisor_vec(<2 x i1> %x, <2 x i8> %y) {
 ; CHECK-LABEL: @urem_with_sext_bool_divisor_vec(
-; CHECK-NEXT:    [[S:%.*]] = sext <2 x i1> [[X:%.*]] to <2 x i8>
-; CHECK-NEXT:    [[REM:%.*]] = urem <2 x i8> [[Y:%.*]], [[S]]
+; CHECK-NEXT:    [[TMP1:%.*]] = icmp eq <2 x i8> [[Y:%.*]], <i8 -1, i8 -1>
+; CHECK-NEXT:    [[REM:%.*]] = select <2 x i1> [[TMP1]], <2 x i8> zeroinitializer, <2 x i8> [[Y]]
 ; CHECK-NEXT:    ret <2 x i8> [[REM]]
 ;
   %s = sext <2 x i1> %x to <2 x i8>