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[SCEV] Make howFarToZero max backedge-taken count check for precondition. 

Refines max backedge-taken count if a loop like
"for (int i = 0; i != n; ++i) { /* body */ }" is rotated.

Differential Revision: https://reviews.llvm.org/D28536

llvm-svn: 291704
This commit is contained in:
Eli Friedman 2017-01-11 21:07:15 +00:00
parent de0f50886f
commit bd6dedaa7f
2 changed files with 19 additions and 4 deletions
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17
llvm/lib/Analysis/ScalarEvolution.cpp
View File

@ -7207,6 +7207,23 @@ ScalarEvolution::howFarToZero(const SCEV *V, const Loop *L, bool ControlsExit,
// N = Distance (as unsigned)
if (StepC->getValue()->equalsInt(1) || StepC->getValue()->isAllOnesValue()) {
APInt MaxBECount = getUnsignedRange(Distance).getUnsignedMax();
// When a loop like "for (int i = 0; i != n; ++i) { /* body */ }" is rotated,
// we end up with a loop whose backedge-taken count is n - 1. Detect this
// case, and see if we can improve the bound.
//
// Explicitly handling this here is necessary because getUnsignedRange
// isn't context-sensitive; it doesn't know that we only care about the
// range inside the loop.
const SCEV *Zero = getZero(Distance->getType());
const SCEV *One = getOne(Distance->getType());
const SCEV *DistancePlusOne = getAddExpr(Distance, One);
if (isLoopEntryGuardedByCond(L, ICmpInst::ICMP_NE, DistancePlusOne, Zero)) {
// If Distance + 1 doesn't overflow, we can compute the maximum distance
// as "unsigned_max(Distance + 1) - 1".
ConstantRange CR = getUnsignedRange(DistancePlusOne);
MaxBECount = APIntOps::umin(MaxBECount, CR.getUnsignedMax() - 1);
}
return ExitLimit(Distance, getConstant(MaxBECount), false, Predicates);
}

6
llvm/test/Analysis/ScalarEvolution/max-trip-count.ll
View File

@ -242,9 +242,8 @@ bar.exit:
ret i32 0
}
; TODO: Improve count based on guard.
; CHECK-LABEL: @ne_max_trip_count_3
; CHECK: Loop %for.body: max backedge-taken count is -1
; CHECK: Loop %for.body: max backedge-taken count is 6
define i32 @ne_max_trip_count_3(i32 %n) {
entry:
%masked = and i32 %n, 7
@ -267,9 +266,8 @@ exit:
ret i32 0
}
; TODO: Improve count based on guard.
; CHECK-LABEL: @ne_max_trip_count_4
; CHECK: Loop %for.body: max backedge-taken count is -1
; CHECK: Loop %for.body: max backedge-taken count is -2
define i32 @ne_max_trip_count_4(i32 %n) {
entry:
%guard = icmp eq i32 %n, 0