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Have GVN also do condition propagation when the right-hand side is not

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a constant.  This fixes PR1768.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@151713 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Duncan Sands
2012-02-29 11:12:03 +00:00
parent 84a1d2371c
commit 2b4f491045
2 changed files with 76 additions and 10 deletions
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29
lib/Transforms/Scalar/GVN.cpp
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@@ -1972,21 +1972,30 @@ bool GVN::propagateEquality(Value *LHS, Value *RHS, BasicBlock *Root) {
if (isa<Constant>(LHS) && isa<Constant>(RHS))
return false;
// Make sure that any constants are on the right-hand side. In general the
// best results are obtained by placing the longest lived value on the RHS.
if (isa<Constant>(LHS))
// Prefer a constant on the right-hand side, or an Argument if no constants.
if (isa<Constant>(LHS) || (isa<Argument>(LHS) && !isa<Constant>(RHS)))
std::swap(LHS, RHS);
assert((isa<Argument>(LHS) || isa<Instruction>(LHS)) && "Unexpected value!");
// If neither term is constant then bail out. This is not for correctness,
// it's just that the non-constant case is much less useful: it occurs just
// as often as the constant case but handling it hardly ever results in an
// improvement.
if (!isa<Constant>(RHS))
return false;
// If there is no obvious reason to prefer the left-hand side over the right-
// hand side, ensure the longest lived term is on the right-hand side, so the
// shortest lived term will be replaced by the longest lived. This tends to
// expose more simplifications.
uint32_t LVN = VN.lookup_or_add(LHS);
if ((isa<Argument>(LHS) && isa<Argument>(RHS)) ||
(isa<Instruction>(LHS) && isa<Instruction>(RHS))) {
// Move the 'oldest' value to the right-hand side, using the value number as
// a proxy for age.
uint32_t RVN = VN.lookup_or_add(RHS);
if (LVN < RVN) {
std::swap(LHS, RHS);
LVN = RVN;
}
}
// If value numbering later deduces that an instruction in the scope is equal
// to 'LHS' then ensure it will be turned into 'RHS'.
addToLeaderTable(VN.lookup_or_add(LHS), RHS, Root);
addToLeaderTable(LVN, RHS, Root);
// Replace all occurrences of 'LHS' with 'RHS' everywhere in the scope. As
// LHS always has at least one use that is not dominated by Root, this will