Teach InstCombine to work with smaller legal types in icmp (shl %v, C1), C2
It enables to work with a smaller constant, which is target friendly for those which can compare to immediates.
It also avoids inserting a shift in favor of a trunc, which can be free on some targets.
This used to work until LLVM-3.1, but regressed with the 3.2 release.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@175270 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Transforms/InstCombine/InstCombineCompares.cpp b/lib/Transforms/InstCombine/InstCombineCompares.cpp
index 40e559e..2e7bd92 100644
--- a/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -1331,6 +1331,25 @@
return new ICmpInst(TrueIfSigned ? ICmpInst::ICMP_NE : ICmpInst::ICMP_EQ,
And, Constant::getNullValue(And->getType()));
}
+
+ // 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.
+ // 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) {
+ Type *NTy = IntegerType::get(ICI.getContext(), TypeBits - Amt);
+ Constant *NCI = ConstantExpr::getTrunc(
+ ConstantExpr::getAShr(RHS,
+ ConstantInt::get(RHS->getType(), Amt)),
+ NTy);
+ return new ICmpInst(ICI.getPredicate(),
+ Builder->CreateTrunc(LHSI->getOperand(0), NTy),
+ ConstantExpr::getTrunc(NCI, NTy));
+ }
+
break;
}