[llvm] r292440 - [InstCombine] add an assert to make a shl+icmp transform assumption explicit; NFCI

[Sanjay Patel via llvm-commits]

Author: spatel
Date: Wed Jan 18 15:16:12 2017
New Revision: 292440

URL: http://llvm.org/viewvc/llvm-project?rev=292440&view=rev
Log:
[InstCombine] add an assert to make a shl+icmp transform assumption explicit; NFCI

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=292440&r1=292439&r2=292440&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/InstCombine/InstCombineCompares.cpp (original)
+++ llvm/trunk/lib/Transforms/InstCombine/InstCombineCompares.cpp Wed Jan 18 15:16:12 2017
@@ -1921,11 +1921,19 @@ Instruction *InstCombiner::foldICmpShlCo
   // A 'shl nuw' is just shifting out zeros, so adjust the compare constant
   // and eliminate the shift.
   if (Shl->hasNoUnsignedWrap()) {
-    if (Cmp.isEquality() || Pred == ICmpInst::ICMP_UGT) {
+    if (Pred == ICmpInst::ICMP_UGT) {
       // icmp Pred (shl nuw X, ShiftAmt), C --> icmp Pred X, (C >>u ShiftAmt)
       APInt ShiftedC = C->lshr(*ShiftAmt);
       return new ICmpInst(Pred, X, ConstantInt::get(ShType, ShiftedC));
     }
+    if (Pred == ICmpInst::ICMP_EQ || Pred == ICmpInst::ICMP_NE) {
+      // This is the same code as the UGT case, but assert the pre-condition
+      // that is needed for this to work with equality predicates.
+      assert(C->lshr(*ShiftAmt).shl(*ShiftAmt) == *C &&
+             "Compare known true or false was not folded");
+      APInt ShiftedC = C->lshr(*ShiftAmt);
+      return new ICmpInst(Pred, X, ConstantInt::get(ShType, ShiftedC));
+    }
     if (Pred == ICmpInst::ICMP_ULT) {
       // ULE is the same as above, but ULE is canonicalized to ULT, so convert:
       // (X << S) <=u C is equiv to X <=u (C >> S) for all C