[llvm-commits] [llvm] r120049 - in /llvm/trunk: lib/Target/README.txt lib/Transforms/InstCombine/InstCombineShifts.cpp test/Transforms/InstCombine/shift.ll

[Benjamin Kramer]

Author: d0k
Date: Tue Nov 23 14:33:57 2010
New Revision: 120049

URL: http://llvm.org/viewvc/llvm-project?rev=120049&view=rev
Log:
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.

Modified:
    llvm/trunk/lib/Target/README.txt
    llvm/trunk/lib/Transforms/InstCombine/InstCombineShifts.cpp
    llvm/trunk/test/Transforms/InstCombine/shift.ll

Modified: llvm/trunk/lib/Target/README.txt
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/README.txt?rev=120049&r1=120048&r2=120049&view=diff
==============================================================================
--- llvm/trunk/lib/Target/README.txt (original)
+++ llvm/trunk/lib/Target/README.txt Tue Nov 23 14:33:57 2010
@@ -1704,8 +1704,8 @@
   %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.
 
 //===---------------------------------------------------------------------===//
 

Modified: llvm/trunk/lib/Transforms/InstCombine/InstCombineShifts.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/InstCombine/InstCombineShifts.cpp?rev=120049&r1=120048&r2=120049&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/InstCombine/InstCombineShifts.cpp (original)
+++ llvm/trunk/lib/Transforms/InstCombine/InstCombineShifts.cpp Tue Nov 23 14:33:57 2010
@@ -54,19 +54,17 @@
     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;
 }

Modified: llvm/trunk/test/Transforms/InstCombine/shift.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstCombine/shift.ll?rev=120049&r1=120048&r2=120049&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/InstCombine/shift.ll (original)
+++ llvm/trunk/test/Transforms/InstCombine/shift.ll Tue Nov 23 14:33:57 2010
@@ -443,12 +443,12 @@
 }
 
 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
 }