[llvm-commits] [llvm] r73540 - in /llvm/trunk: lib/Analysis/ScalarEvolution.cpp test/Transforms/IndVarSimplify/shrunk-constant.ll

[Dan Gohman]

Author: djg
Date: Tue Jun 16 14:52:01 2009
New Revision: 73540

URL: http://llvm.org/viewvc/llvm-project?rev=73540&view=rev
Log:
Instcombine's ShrinkDemandedConstant may strip bits out of constants,
obscuring what would otherwise be a low-bits mask.  Use ComputeMaskedBits
to compute what ShrinkDemandedConstant knew about to reconstruct a
low-bits mask value.

Added:
    llvm/trunk/test/Transforms/IndVarSimplify/shrunk-constant.ll
Modified:
    llvm/trunk/lib/Analysis/ScalarEvolution.cpp

Modified: llvm/trunk/lib/Analysis/ScalarEvolution.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/ScalarEvolution.cpp?rev=73540&r1=73539&r2=73540&view=diff

==============================================================================
--- llvm/trunk/lib/Analysis/ScalarEvolution.cpp (original)
+++ llvm/trunk/lib/Analysis/ScalarEvolution.cpp Tue Jun 16 14:52:01 2009
@@ -68,6 +68,7 @@
 #include "llvm/Analysis/ConstantFolding.h"
 #include "llvm/Analysis/Dominators.h"
 #include "llvm/Analysis/LoopInfo.h"
+#include "llvm/Analysis/ValueTracking.h"
 #include "llvm/Assembly/Writer.h"
 #include "llvm/Target/TargetData.h"
 #include "llvm/Support/CommandLine.h"
@@ -2394,14 +2395,28 @@
       if (CI->isAllOnesValue())
         return getSCEV(U->getOperand(0));
       const APInt &A = CI->getValue();
-      unsigned Ones = A.countTrailingOnes();
-      if (APIntOps::isMask(Ones, A))
+
+      // Instcombine's ShrinkDemandedConstant may strip bits out of
+      // constants, obscuring what would otherwise be a low-bits mask.
+      // Use ComputeMaskedBits to compute what ShrinkDemandedConstant
+      // knew about to reconstruct a low-bits mask value.
+      unsigned LZ = A.countLeadingZeros();
+      unsigned BitWidth = A.getBitWidth();
+      APInt AllOnes = APInt::getAllOnesValue(BitWidth);
+      APInt KnownZero(BitWidth, 0), KnownOne(BitWidth, 0);
+      ComputeMaskedBits(U->getOperand(0), AllOnes, KnownZero, KnownOne, TD);
+
+      APInt EffectiveMask = APInt::getLowBitsSet(BitWidth, BitWidth - LZ);
+
+      if (LZ != 0 && !((~A & ~KnownZero) & EffectiveMask)) {
         return
           getZeroExtendExpr(getTruncateExpr(getSCEV(U->getOperand(0)),
-                                            IntegerType::get(Ones)),
+                                            IntegerType::get(BitWidth - LZ)),
                             U->getType());
+      }
     }
     break;
+
   case Instruction::Or:
     // If the RHS of the Or is a constant, we may have something like:
     // X*4+1 which got turned into X*4|1.  Handle this as an Add so loop

Added: llvm/trunk/test/Transforms/IndVarSimplify/shrunk-constant.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/IndVarSimplify/shrunk-constant.ll?rev=73540&view=auto

==============================================================================
--- llvm/trunk/test/Transforms/IndVarSimplify/shrunk-constant.ll (added)
+++ llvm/trunk/test/Transforms/IndVarSimplify/shrunk-constant.ll Tue Jun 16 14:52:01 2009
@@ -0,0 +1,14 @@
+; RUN: llvm-as < %s | opt -iv-users -analyze | grep store
+
+define fastcc void @foo() nounwind {
+entry:
+	br label %loop
+
+loop:
+	%i = phi i32 [ 0, %entry ], [ %t2, %loop ]
+	%t0 = add i32 %i, 9
+	%t1 = and i32 %t0, 9
+        store i32 %t1, i32* null
+	%t2 = add i32 %i, 8
+	br label %loop
+}