Optimize integral reciprocal (udiv 1, x and sdiv 1, x) to not use div…

…ision. This fires exactly once in a clang bootstrap, but covers a few different results from http://www.cs.utah.edu/~regehr/souper/ git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208750 91177308-0d34-0410-b5e6-96231b3b80d8
tzlaine · May 14, 2014 · 8f84449 · 8f84449
1 parent ee8af3e
commit 8f84449
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Showing 2 changed files with 39 additions and 1 deletion.
diff --git a/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp b/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
@@ -727,7 +727,7 @@ Instruction *InstCombiner::commonIDivTransforms(BinaryOperator &I) {
       if (Instruction::BinaryOps(LHS->getOpcode()) == I.getOpcode())
         if (ConstantInt *LHSRHS = dyn_cast<ConstantInt>(LHS->getOperand(1))) {
           if (MultiplyOverflows(RHS, LHSRHS,
-                                I.getOpcode()==Instruction::SDiv))
+                                I.getOpcode() == Instruction::SDiv))
             return ReplaceInstUsesWith(I, Constant::getNullValue(I.getType()));
           return BinaryOperator::Create(I.getOpcode(), LHS->getOperand(0),
                                         ConstantExpr::getMul(RHS, LHSRHS));
@@ -743,6 +743,25 @@ Instruction *InstCombiner::commonIDivTransforms(BinaryOperator &I) {
     }
   }
 
+  if (ConstantInt *One = dyn_cast<ConstantInt>(Op0)) {
+    if (One->isOne() && !I.getType()->isIntegerTy(1)) {
+      bool isSigned = I.getOpcode() == Instruction::SDiv;
+      if (isSigned) {
+        // If Op1 is 0 then it's undefined behaviour, if Op1 is 1 then the
+        // result is one, if Op1 is -1 then the result is minus one, otherwise
+        // it's zero.
+        Value *Inc = Builder->CreateAdd(Op1, One);
+        Value *Cmp = Builder->CreateICmpULT(
+                         Inc, ConstantInt::get(I.getType(), 3));
+        return SelectInst::Create(Cmp, Op1, ConstantInt::get(I.getType(), 0));
+      } else {
+        // If Op1 is 0 then it's undefined behaviour. If Op1 is 1 then the
+        // result is one, otherwise it's zero.
+        return new ZExtInst(Builder->CreateICmpEQ(Op1, One), I.getType());
+      }
+    }
+  }
+
   // See if we can fold away this div instruction.
   if (SimplifyDemandedInstructionBits(I))
     return &I;

diff --git a/test/Transforms/InstCombine/div.ll b/test/Transforms/InstCombine/div.ll
@@ -156,3 +156,22 @@ define <2 x i64> @test18(<2 x i64> %x) nounwind {
 ; CHECK-NEXT: sub <2 x i64> zeroinitializer, %x
 ; CHECK-NEXT: ret <2 x i64>
 }
+
+define i32 @test19(i32 %x) {
+  %A = udiv i32 1, %x
+  ret i32 %A
+; CHECK-LABEL: @test19(
+; CHECK-NEXT: icmp eq i32 %x, 1
+; CHECK-NEXT: zext i1 %{{.*}} to i32
+; CHECK-NEXT ret i32
+}
+
+define i32 @test20(i32 %x) {
+  %A = sdiv i32 1, %x
+  ret i32 %A
+; CHECK-LABEL: @test20(
+; CHECK-NEXT: add i32 %x, 1
+; CHECK-NEXT: icmp ult i32 %{{.*}}, 3
+; CHECK-NEXT: select i1 %{{.*}}, i32 %x, i32 {{.*}}
+; CHECK-NEXT: ret i32
+}