The srem -> urem transform is not safe for any divisor that's not a power of two.

E.g. -5 % 5 is 0 with srem and 1 with urem.

Also addresses Frits van Bommel's comments.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@120049 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Target/README.txt b/lib/Target/README.txt
index b98531ca85920767d29d921270e9dd7d3e126103..38f5af642d7172384cec3e11aef738fd579fe801 100644 (file)
--- a/lib/Target/README.txt
+++ b/lib/Target/README.txt
@@ -1704,8 +1704,8 @@ Missed instcombine transformation:
   %384 = shl i64 %381, %383                       ; [#uses=1]
   %385 = icmp slt i32 %tmp14.i, 64                ; [#uses=1]
 
-The srem can be transformed to an and because if x is negative, the shift is
-undefined.  Testcase derived from 403.gcc.
+The srem can be transformed to an and because if %tmp14.i is negative, the
+shift is undefined.  Testcase derived from 403.gcc.
 
 //===---------------------------------------------------------------------===//
 

diff --git a/lib/Transforms/InstCombine/InstCombineShifts.cpp b/lib/Transforms/InstCombine/InstCombineShifts.cpp
index 7969dbfec04b7abe7c7c8bb11f8a9fe1a20d0059..e52f2013fe50a5650d99845d006a4b46a53cd899 100644 (file)
--- a/lib/Transforms/InstCombine/InstCombineShifts.cpp
+++ b/lib/Transforms/InstCombine/InstCombineShifts.cpp
@@ -54,19 +54,17 @@ Instruction *InstCombiner::commonShiftTransforms(BinaryOperator &I) {
     if (Instruction *Res = FoldShiftByConstant(Op0, CUI, I))
       return Res;
 
-  // X shift (A srem B) -> X shift (A urem B) iff B is positive.
+  // X shift (A srem B) -> X shift (A and B-1) iff B is a power of 2.
   // 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;
-      }
-    }
+    if (BO->hasOneUse() && BO->getOpcode() == Instruction::SRem)
+      if (ConstantInt *CI = dyn_cast<ConstantInt>(BO->getOperand(1)))
+        if (CI->getValue().isPowerOf2()) {
+          Constant *C = ConstantInt::get(BO->getType(), CI->getValue()-1);
+          Value *Rem = Builder->CreateAnd(BO->getOperand(0), C, BO->getName());
+          I.setOperand(1, Rem);
+          return &I;
+        }
 
   return 0;
 }

diff --git a/test/Transforms/InstCombine/shift.ll b/test/Transforms/InstCombine/shift.ll
index 6bebca9cc85f6ea79662eff5cbf8dcead99605ba..8d1c82991fdbef857d08cd2d287d259f22e5e5d2 100644 (file)
--- a/test/Transforms/InstCombine/shift.ll
+++ b/test/Transforms/InstCombine/shift.ll
@@ -443,12 +443,12 @@ entry:
 }
 
 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
+; CHECK-NEXT: and i32 %x, 31
+; CHECK-NEXT: shl i32 1
+; CHECK-NEXT: ret i32
 }