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Fix bug: test/Regression/Transforms/GCSE/2002-05-21-NoSharedDominator.ll

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llvm-svn: 3215
This commit is contained in:
Chris Lattner 2002-08-02 18:06:01 +00:00
parent e980c93ebc
commit b55222222e
1 changed files with 29 additions and 0 deletions
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29
lib/Transforms/Scalar/GCSE.cpp
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@ -192,6 +192,34 @@ void GCSE::CommonSubExpressionFound(Instruction *I, Instruction *Other) {
} else if (DomSetInfo->dominates(BB2, BB1)) { // Other dom I?
ReplaceInstWithInst(Other, I);
} else {
// This code is disabled because it has several problems:
// One, the actual assumption is wrong, as shown by this code:
// int "test"(int %X, int %Y) {
// %Z = add int %X, %Y
// ret int %Z
// Unreachable:
// %Q = add int %X, %Y
// ret int %Q
// }
//
// Here there are no shared dominators. Additionally, this had the habit of
// moving computations where they were not always computed. For example, in
// a cast like this:
// if (c) {
// if (d) ...
// else ... X+Y ...
// } else {
// ... X+Y ...
// }
//
// In thiscase, the expression would be hoisted to outside the 'if' stmt,
// causing the expression to be evaluated, even for the if (d) path, which
// could cause problems, if, for example, it caused a divide by zero. In
// general the problem this case is trying to solve is better addressed with
// PRE than GCSE.
//
#if 0
// Handle the most general case now. In this case, neither I dom Other nor
// Other dom I. Because we are in SSA form, we are guaranteed that the
// operands of the two instructions both dominate the uses, so we _know_
@ -215,6 +243,7 @@ void GCSE::CommonSubExpressionFound(Instruction *I, Instruction *Other) {
// Eliminate 'Other' now.
ReplaceInstWithInst(I, Other);
#endif
}
}