[llvm] r222230 - InstCombine: Fold away tautological masked compares

[David Majnemer]

Author: majnemer
Date: Tue Nov 18 03:31:41 2014
New Revision: 222230

URL: http://llvm.org/viewvc/llvm-project?rev=222230&view=rev
Log:
InstCombine: Fold away tautological masked compares

It is impossible for (x & INT_MAX) == 0 && x == INT_MAX to ever be true.

While this sort of reasoning should normally live in InstSimplify,
the machinery that derives this result is not trivial to split out.

Modified:
    llvm/trunk/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
    llvm/trunk/test/Transforms/InstCombine/icmp-logical.ll

Modified: llvm/trunk/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp?rev=222230&r1=222229&r2=222230&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp (original)
+++ llvm/trunk/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp Tue Nov 18 03:31:41 2014
@@ -776,7 +776,7 @@ static Value *foldLogOpOfMaskedICmps(ICm
     // whole construct
     if (((BCst->getValue() & DCst->getValue()) &
          (CCst->getValue() ^ ECst->getValue())) != 0)
-      return nullptr;
+      return ConstantInt::get(LHS->getType(), !IsAnd);
     Value *newOr1 = Builder->CreateOr(B, D);
     Value *newOr2 = ConstantExpr::getOr(CCst, ECst);
     Value *newAnd = Builder->CreateAnd(A, newOr1);

Modified: llvm/trunk/test/Transforms/InstCombine/icmp-logical.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstCombine/icmp-logical.ll?rev=222230&r1=222229&r2=222230&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/InstCombine/icmp-logical.ll (original)
+++ llvm/trunk/test/Transforms/InstCombine/icmp-logical.ll Tue Nov 18 03:31:41 2014
@@ -150,3 +150,23 @@ define i1 @nomask_rhs(i32 %in) {
   %val = or i1 %tst1, %tst2
   ret i1 %val
 }
+
+define i1 @fold_mask_cmps_to_false(i32 %x) {
+; CHECK-LABEL: @fold_mask_cmps_to_false
+; CHECK: ret i1 false
+  %1 = and i32 %x, 2147483647
+  %2 = icmp eq i32 %1, 0
+  %3 = icmp eq i32 %x, 2147483647
+  %4 = and i1 %3, %2
+  ret i1 %4
+}
+
+define i1 @fold_mask_cmps_to_true(i32 %x) {
+; CHECK-LABEL: @fold_mask_cmps_to_true
+; CHECK: ret i1 true
+  %1 = and i32 %x, 2147483647
+  %2 = icmp ne i32 %1, 0
+  %3 = icmp ne i32 %x, 2147483647
+  %4 = or i1 %3, %2
+  ret i1 %4
+}