[llvm] r176950 - Fix a performance regression when combining to smaller types in icmp (shl %v, C1), C2 :

[Arnaud A. de Grandmaison]

Author: aadg
Date: Wed Mar 13 09:40:37 2013
New Revision: 176950

URL: http://llvm.org/viewvc/llvm-project?rev=176950&view=rev
Log:
Fix a performance regression when combining to smaller types in icmp (shl %v, C1), C2 :

Only combine when the shl is only used by the icmp

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

Modified: llvm/trunk/lib/Transforms/InstCombine/InstCombineCompares.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/InstCombine/InstCombineCompares.cpp?rev=176950&r1=176949&r2=176950&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/InstCombine/InstCombineCompares.cpp (original)
+++ llvm/trunk/lib/Transforms/InstCombine/InstCombineCompares.cpp Wed Mar 13 09:40:37 2013
@@ -1333,13 +1333,14 @@ Instruction *InstCombiner::visitICmpInst
     }
 
     // Transform (icmp pred iM (shl iM %v, N), CI)
-    // -> (icmp pred i(M-N) (trunc %v iM to i(N-N)), (trunc (CI>>N))
-    // Transform the shl to a trunc if (trunc (CI>>N)) has no loss.
+    // -> (icmp pred i(M-N) (trunc %v iM to i(M-N)), (trunc (CI>>N))
+    // Transform the shl to a trunc if (trunc (CI>>N)) has no loss and M-N.
     // This enables to get rid of the shift in favor of a trunc which can be
     // free on the target. It has the additional benefit of comparing to a
     // smaller constant, which will be target friendly.
     unsigned Amt = ShAmt->getLimitedValue(TypeBits-1);
-    if (Amt != 0 && RHSV.countTrailingZeros() >= Amt) {
+    if (LHSI->hasOneUse() &&
+        Amt != 0 && RHSV.countTrailingZeros() >= Amt) {
       Type *NTy = IntegerType::get(ICI.getContext(), TypeBits - Amt);
       Constant *NCI = ConstantExpr::getTrunc(
                         ConstantExpr::getAShr(RHS,