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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.

llvm-svn: 222230
This commit is contained in:
David Majnemer 2014-11-18 09:31:41 +00:00
parent 1a3327bb62
commit 6fdb6b8fd4
2 changed files with 21 additions and 1 deletions
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2
llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
View File

@ -776,7 +776,7 @@ static Value *foldLogOpOfMaskedICmps(ICmpInst *LHS, ICmpInst *RHS, bool IsAnd,
// whole construct // whole construct
if (((BCst->getValue() & DCst->getValue()) & if (((BCst->getValue() & DCst->getValue()) &
(CCst->getValue() ^ ECst->getValue())) != 0) (CCst->getValue() ^ ECst->getValue())) != 0)
return nullptr; return ConstantInt::get(LHS->getType(), !IsAnd);
Value *newOr1 = Builder->CreateOr(B, D); Value *newOr1 = Builder->CreateOr(B, D);
Value *newOr2 = ConstantExpr::getOr(CCst, ECst); Value *newOr2 = ConstantExpr::getOr(CCst, ECst);
Value *newAnd = Builder->CreateAnd(A, newOr1); Value *newAnd = Builder->CreateAnd(A, newOr1);

20
llvm/test/Transforms/InstCombine/icmp-logical.ll
View File

@ -150,3 +150,23 @@ define i1 @nomask_rhs(i32 %in) {
%val = or i1 %tst1, %tst2 %val = or i1 %tst1, %tst2
ret i1 %val 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
}