InstCombine: Reduce "X shift (A srem B)" to "X shift (A urem B)" iff B is positive.

This allows to transform the rem in "1 << ((int)x % 8);" to an and.

llvm-svn: 120028

llvm/lib/Target/README.txt

@@ -1698,22 +1698,14 @@ from gcc.
 //===---------------------------------------------------------------------===//
 
 Missed instcombine transformation:
-define i32 @a(i32 %x) nounwind readnone {
-entry:
-  %rem = srem i32 %x, 32
-  %shl = shl i32 1, %rem
-  ret i32 %shl
-}
-
-The srem can be transformed to an and because if x is negative, the shift is
-undefined.  Here's a more general case that occurs on 64-bit targets:
 
   %382 = srem i32 %tmp14.i, 64                    ; [#uses=1]
   %383 = zext i32 %382 to i64                     ; [#uses=1]
   %384 = shl i64 %381, %383                       ; [#uses=1]
   %385 = icmp slt i32 %tmp14.i, 64                ; [#uses=1]
 
-Testcase derived from 403.gcc.
+The srem can be transformed to an and because if x is negative, the shift is
+undefined.  Testcase derived from 403.gcc.
 
 //===---------------------------------------------------------------------===//
 

llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp

@@ -53,6 +53,21 @@ Instruction *InstCombiner::commonShiftTransforms(BinaryOperator &I) {
   if (ConstantInt *CUI = dyn_cast<ConstantInt>(Op1))
     if (Instruction *Res = FoldShiftByConstant(Op0, CUI, I))
       return Res;
+
+  // X shift (A srem B) -> X shift (A urem B) iff B is positive.
+  // Because shifts by negative values are undefined.
+  if (BinaryOperator *BO = dyn_cast<BinaryOperator>(Op1))
+    if (BO->getOpcode() == Instruction::SRem && BO->getType()->isIntegerTy()) {
+      // Make sure the divisor's sign bit is zero.
+      APInt Mask = APInt::getSignBit(BO->getType()->getPrimitiveSizeInBits());
+      if (MaskedValueIsZero(BO->getOperand(1), Mask)) {
+        Value *URem = Builder->CreateURem(BO->getOperand(0), BO->getOperand(1),
+                                          BO->getName());
+        I.setOperand(1, URem);
+        return &I;
+      }
+    }
+
   return 0;
 }
 

llvm/test/Transforms/InstCombine/shift.ll

@@ -441,3 +441,14 @@ entry:
 ; CHECK:  %ins = or i128 %tmp23, %A
 ; CHECK:  %tmp46 = trunc i128 %ins to i64
 }
+
+define i32 @test38(i32 %x) nounwind readnone {
+entry:
+  %rem = srem i32 %x, 32
+  %shl = shl i32 1, %rem
+  ret i32 %shl
+; CHECK: @test38
+; CHECK-NOT: srem
+; CHECK: ret i32
+}
+