[llvm-commits] [llvm] r128744 - /llvm/trunk/lib/Transforms/InstCombine/InstCombineCasts.cpp

[Benjamin Kramer]

Author: d0k
Date: Fri Apr  1 17:22:11 2011
New Revision: 128744

URL: http://llvm.org/viewvc/llvm-project?rev=128744&view=rev
Log:
Tweaks to the icmp+sext-to-shifts optimization to address Frits' comments:

- Localize the check if an icmp has one use to a place where we know we're
  introducing something that's likely more expensive than a sext from i1.
- Add an assert to make sure a case that would lead to a miscompilation is
  folded away earlier.
- Fix a typo.

Modified:
    llvm/trunk/lib/Transforms/InstCombine/InstCombineCasts.cpp

Modified: llvm/trunk/lib/Transforms/InstCombine/InstCombineCasts.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/InstCombine/InstCombineCasts.cpp?rev=128744&r1=128743&r2=128744&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/InstCombine/InstCombineCasts.cpp (original)
+++ llvm/trunk/lib/Transforms/InstCombine/InstCombineCasts.cpp Fri Apr  1 17:22:11 2011
@@ -882,10 +882,6 @@
   Value *Op0 = ICI->getOperand(0), *Op1 = ICI->getOperand(1);
   ICmpInst::Predicate Pred = ICI->getPredicate();
 
-  // Transforming icmps with more than one use is not profitable.
-  if (!ICI->hasOneUse())
-    return 0;
-
   if (ConstantInt *Op1C = dyn_cast<ConstantInt>(Op1)) {
     // (x <s 0) ? -1 : 0 -> ashr x, 31   -> all ones if signed
     // (x >s -1) ? -1 : 0 -> ashr x, 31  -> all ones if not signed
@@ -906,7 +902,8 @@
     // If we know that only one bit of the LHS of the icmp can be set and we
     // have an equality comparison with zero or a power of 2, we can transform
     // the icmp and sext into bitwise/integer operations.
-    if (ICI->isEquality() && (Op1C->isZero() || Op1C->getValue().isPowerOf2())){
+    if (ICI->hasOneUse() &&
+        ICI->isEquality() && (Op1C->isZero() || Op1C->getValue().isPowerOf2())){
       unsigned BitWidth = Op1C->getType()->getBitWidth();
       APInt KnownZero(BitWidth, 0), KnownOne(BitWidth, 0);
       APInt TypeMask(APInt::getAllOnesValue(BitWidth));
@@ -916,6 +913,9 @@
       if (KnownZeroMask.isPowerOf2()) {
         Value *In = ICI->getOperand(0);
 
+        assert((Op1C->isZero() || Op1C->getValue() == KnownZeroMask) &&
+               "Constant icmp not folded?");
+
         if (!Op1C->isZero() == (Pred == ICmpInst::ICMP_NE)) {
           // sext ((x & 2^n) == 0)   -> (x >> n) - 1
           // sext ((x & 2^n) != 2^n) -> (x >> n) - 1
@@ -932,7 +932,7 @@
                                   "sext");
         } else {
           // sext ((x & 2^n) != 0)   -> (x << bitwidth-n) a>> bitwidth-1
-          // sext ((x & 2^n) != 2^n) -> (x << bitwidth-n) a>> bitwidth-1
+          // sext ((x & 2^n) == 2^n) -> (x << bitwidth-n) a>> bitwidth-1
           unsigned ShiftAmt = KnownZeroMask.countLeadingZeros();
           // Perform a left shift to place the desired bit in the MSB.
           if (ShiftAmt)