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indvars: Added isHighCostExpansion. Avoid generating extra ops in the

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preheader for the sole purpose of LFTR, since LFTR itself is usually not
a clear optimization.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@135409 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Andrew Trick 2011-07-18 18:21:35 +00:00
parent 3f6a8dd4ce
commit 5241b79ebc
1 changed files with 51 additions and 17 deletions
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68
lib/Transforms/Scalar/IndVarSimplify.cpp
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@ -1448,6 +1448,54 @@ void IndVarSimplify::SimplifyCongruentIVs(Loop *L) {
// LinearFunctionTestReplace and its kin. Rewrite the loop exit condition.
//===----------------------------------------------------------------------===//
// Check for expressions that ScalarEvolution generates to compute
// BackedgeTakenInfo. If these expressions have not been reduced, then expanding
// them may incur additional cost (albeit in the loop preheader).
static bool isHighCostExpansion(const SCEV *S, BranchInst *BI,
ScalarEvolution *SE) {
// If the backedge-taken count is a UDiv, it's very likely a UDiv that
// ScalarEvolution's HowFarToZero or HowManyLessThans produced to compute a
// precise expression, rather than a UDiv from the user's code. If we can't
// find a UDiv in the code with some simple searching, assume the former and
// forego rewriting the loop.
if (isa<SCEVUDivExpr>(S)) {
ICmpInst *OrigCond = dyn_cast<ICmpInst>(BI->getCondition());
if (!OrigCond) return true;
const SCEV *R = SE->getSCEV(OrigCond->getOperand(1));
R = SE->getMinusSCEV(R, SE->getConstant(R->getType(), 1));
if (R != S) {
const SCEV *L = SE->getSCEV(OrigCond->getOperand(0));
L = SE->getMinusSCEV(L, SE->getConstant(L->getType(), 1));
if (L != S)
return true;
}
}
if (!DisableIVRewrite)
return false;
// Recurse past add expressions, which commonly occur in the
// BackedgeTakenCount. They may already exist in program code, and if not,
// they are not too expensive rematerialize.
if (const SCEVAddExpr *Add = dyn_cast<SCEVAddExpr>(S)) {
for (SCEVAddExpr::op_iterator I = Add->op_begin(), E = Add->op_end();
I != E; ++I) {
if (isHighCostExpansion(*I, BI, SE))
return true;
}
return false;
}
// HowManyLessThans uses a Max expression whenever the loop is not guarded by
// the exit condition.
if (isa<SCEVSMaxExpr>(S) || isa<SCEVUMaxExpr>(S))
return true;
// If we haven't recognized an expensive SCEV patter, assume its an expression
// produced by program code.
return false;
}
/// canExpandBackedgeTakenCount - Return true if this loop's backedge taken
/// count expression can be safely and cheaply expanded into an instruction
/// sequence that can be used by LinearFunctionTestReplace.
@ -1465,23 +1513,9 @@ static bool canExpandBackedgeTakenCount(Loop *L, ScalarEvolution *SE) {
if (!BI)
return false;
// Special case: If the backedge-taken count is a UDiv, it's very likely a
// UDiv that ScalarEvolution produced in order to compute a precise
// expression, rather than a UDiv from the user's code. If we can't find a
// UDiv in the code with some simple searching, assume the former and forego
// rewriting the loop.
if (isa<SCEVUDivExpr>(BackedgeTakenCount)) {
ICmpInst *OrigCond = dyn_cast<ICmpInst>(BI->getCondition());
if (!OrigCond) return false;
const SCEV *R = SE->getSCEV(OrigCond->getOperand(1));
R = SE->getMinusSCEV(R, SE->getConstant(R->getType(), 1));
if (R != BackedgeTakenCount) {
const SCEV *L = SE->getSCEV(OrigCond->getOperand(0));
L = SE->getMinusSCEV(L, SE->getConstant(L->getType(), 1));
if (L != BackedgeTakenCount)
return false;
}
}
if (isHighCostExpansion(BackedgeTakenCount, BI, SE))
return false;
return true;
}