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[LoopPred] Handle a subset of NE comparison based latches 

At the moment, LoopPredication completely bails out if it sees a latch of the form:
%cmp = icmp ne %iv, %N
br i1 %cmp, label %loop, label %exit
OR
%cmp = icmp ne %iv.next, %NPlus1
br i1 %cmp, label %loop, label %exit

This is unfortunate since this is exactly the form that LFTR likes to produce. So, go ahead and recognize simple cases where we can.

For pre-increment loops, we leverage the fact that LFTR likes canonical counters (i.e. those starting at zero) and a (presumed) range fact on RHS to discharge the check trivially.

For post-increment forms, the key insight is in remembering that LFTR had to insert a (N+1) for the RHS. CVP can hopefully prove that add nsw/nuw (if there's appropriate range on N to start with). This leaves us both with the post-inc IV and the RHS involving an nsw/nuw add, and SCEV can discharge that with no problem.

This does still need to be extended to handle non-one steps, or other harder patterns of variable (but range restricted) starting values. That'll come later.

Differential Revision: https://reviews.llvm.org/D62748

llvm-svn: 362282
This commit is contained in:
Philip Reames 2019-06-01 00:31:58 +00:00
parent d8e8722791
commit 099eca832e
2 changed files with 47 additions and 27 deletions
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54
llvm/lib/Transforms/Scalar/LoopPredication.cpp
View File

@ -230,23 +230,23 @@ static cl::opt<bool> PredicateWidenableBranchGuards(
cl::init(true));
namespace {
class LoopPredication {
/// Represents an induction variable check:
/// icmp Pred, <induction variable>, <loop invariant limit>
struct LoopICmp {
ICmpInst::Predicate Pred;
const SCEVAddRecExpr *IV;
const SCEV *Limit;
LoopICmp(ICmpInst::Predicate Pred, const SCEVAddRecExpr *IV,
const SCEV *Limit)
: Pred(Pred), IV(IV), Limit(Limit) {}
LoopICmp() {}
void dump() {
dbgs() << "LoopICmp Pred = " << Pred << ", IV = " << *IV
<< ", Limit = " << *Limit << "\n";
}
};
/// Represents an induction variable check:
/// icmp Pred, <induction variable>, <loop invariant limit>
struct LoopICmp {
ICmpInst::Predicate Pred;
const SCEVAddRecExpr *IV;
const SCEV *Limit;
LoopICmp(ICmpInst::Predicate Pred, const SCEVAddRecExpr *IV,
const SCEV *Limit)
: Pred(Pred), IV(IV), Limit(Limit) {}
LoopICmp() {}
void dump() {
dbgs() << "LoopICmp Pred = " << Pred << ", IV = " << *IV
<< ", Limit = " << *Limit << "\n";
}
};
class LoopPredication {
AliasAnalysis *AA;
ScalarEvolution *SE;
BranchProbabilityInfo *BPI;
@ -382,7 +382,7 @@ PreservedAnalyses LoopPredicationPass::run(Loop &L, LoopAnalysisManager &AM,
return getLoopPassPreservedAnalyses();
}
Optional<LoopPredication::LoopICmp>
Optional<LoopICmp>
LoopPredication::parseLoopICmp(ICmpInst::Predicate Pred, Value *LHS,
Value *RHS) {
const SCEV *LHSS = SE->getSCEV(LHS);
@ -428,7 +428,7 @@ Value *LoopPredication::expandCheck(SCEVExpander &Expander,
return Builder.CreateICmp(Pred, LHSV, RHSV);
}
Optional<LoopPredication::LoopICmp>
Optional<LoopICmp>
LoopPredication::generateLoopLatchCheck(Type *RangeCheckType) {
auto *LatchType = LatchCheck.IV->getType();
@ -518,7 +518,7 @@ bool LoopPredication::isLoopInvariantValue(const SCEV* S) {
}
Optional<Value *> LoopPredication::widenICmpRangeCheckIncrementingLoop(
LoopPredication::LoopICmp LatchCheck, LoopPredication::LoopICmp RangeCheck,
LoopICmp LatchCheck, LoopICmp RangeCheck,
SCEVExpander &Expander, Instruction *Guard) {
auto *Ty = RangeCheck.IV->getType();
// Generate the widened condition for the forward loop:
@ -567,7 +567,7 @@ Optional<Value *> LoopPredication::widenICmpRangeCheckIncrementingLoop(
}
Optional<Value *> LoopPredication::widenICmpRangeCheckDecrementingLoop(
LoopPredication::LoopICmp LatchCheck, LoopPredication::LoopICmp RangeCheck,
LoopICmp LatchCheck, LoopICmp RangeCheck,
SCEVExpander &Expander, Instruction *Guard) {
auto *Ty = RangeCheck.IV->getType();
const SCEV *GuardStart = RangeCheck.IV->getStart();
@ -614,6 +614,17 @@ Optional<Value *> LoopPredication::widenICmpRangeCheckDecrementingLoop(
return Builder.CreateAnd(FirstIterationCheck, LimitCheck);
}
static void normalizePredicate(ScalarEvolution *SE, Loop *L,
LoopICmp& RC) {
// LFTR canonicalizes checks to the ICMP_NE form instead of an ULT/SLT form.
// Normalize back to the ULT/SLT form for ease of handling.
if (RC.Pred == ICmpInst::ICMP_NE &&
RC.IV->getStepRecurrence(*SE)->isOne() &&
SE->isKnownPredicate(ICmpInst::ICMP_ULE, RC.IV->getStart(), RC.Limit))
RC.Pred = ICmpInst::ICMP_ULT;
}
/// If ICI can be widened to a loop invariant condition emits the loop
/// invariant condition in the loop preheader and return it, otherwise
/// returns None.
@ -798,7 +809,7 @@ bool LoopPredication::widenWidenableBranchGuardConditions(
return true;
}
Optional<LoopPredication::LoopICmp> LoopPredication::parseLoopLatchICmp() {
Optional<LoopICmp> LoopPredication::parseLoopLatchICmp() {
using namespace PatternMatch;
BasicBlock *LoopLatch = L->getLoopLatch();
@ -852,6 +863,7 @@ Optional<LoopPredication::LoopICmp> LoopPredication::parseLoopLatchICmp() {
}
};
normalizePredicate(SE, L, *Result);
if (IsUnsupportedPredicate(Step, Result->Pred)) {
LLVM_DEBUG(dbgs() << "Unsupported loop latch predicate(" << Result->Pred
<< ")!\n");

20
llvm/test/Transforms/LoopPredication/basic.ll
View File

@ -1603,11 +1603,13 @@ define i32 @ne_latch_zext(i32* %array, i32 %length, i16 %n16) {
; CHECK-NEXT: loop.preheader:
; CHECK-NEXT: [[N:%.*]] = zext i16 [[N16:%.*]] to i32
; CHECK-NEXT: [[NPLUS1:%.*]] = add nuw nsw i32 [[N]], 1
; CHECK-NEXT: [[TMP0:%.*]] = icmp ule i32 [[NPLUS1]], [[LENGTH:%.*]]
; CHECK-NEXT: [[TMP1:%.*]] = icmp ult i32 0, [[LENGTH]]
; CHECK-NEXT: [[TMP2:%.*]] = and i1 [[TMP1]], [[TMP0]]
; CHECK-NEXT: br label [[LOOP:%.*]]
; CHECK: loop:
; CHECK-NEXT: [[I:%.*]] = phi i32 [ [[I_NEXT:%.*]], [[LOOP]] ], [ 0, [[LOOP_PREHEADER:%.*]] ]
; CHECK-NEXT: [[WITHIN_BOUNDS:%.*]] = icmp ult i32 [[I]], [[LENGTH:%.*]]
; CHECK-NEXT: call void (i1, ...) @llvm.experimental.guard(i1 [[WITHIN_BOUNDS]], i32 9) [ "deopt"() ]
; CHECK-NEXT: call void (i1, ...) @llvm.experimental.guard(i1 [[TMP2]], i32 9) [ "deopt"() ]
; CHECK-NEXT: [[I_NEXT]] = add nuw nsw i32 [[I]], 1
; CHECK-NEXT: [[CONTINUE:%.*]] = icmp ne i32 [[I_NEXT]], [[NPLUS1]]
; CHECK-NEXT: br i1 [[CONTINUE]], label [[LOOP]], label [[EXIT:%.*]]
@ -1637,11 +1639,14 @@ define i32 @ne_latch_zext_preinc(i32* %array, i32 %length, i16 %n16) {
; CHECK-LABEL: @ne_latch_zext_preinc(
; CHECK-NEXT: loop.preheader:
; CHECK-NEXT: [[N:%.*]] = zext i16 [[N16:%.*]] to i32
; CHECK-NEXT: [[TMP0:%.*]] = add i32 [[LENGTH:%.*]], -1
; CHECK-NEXT: [[TMP1:%.*]] = icmp ule i32 [[N]], [[TMP0]]
; CHECK-NEXT: [[TMP2:%.*]] = icmp ult i32 0, [[LENGTH]]
; CHECK-NEXT: [[TMP3:%.*]] = and i1 [[TMP2]], [[TMP1]]
; CHECK-NEXT: br label [[LOOP:%.*]]
; CHECK: loop:
; CHECK-NEXT: [[I:%.*]] = phi i32 [ [[I_NEXT:%.*]], [[LOOP]] ], [ 0, [[LOOP_PREHEADER:%.*]] ]
; CHECK-NEXT: [[WITHIN_BOUNDS:%.*]] = icmp ult i32 [[I]], [[LENGTH:%.*]]
; CHECK-NEXT: call void (i1, ...) @llvm.experimental.guard(i1 [[WITHIN_BOUNDS]], i32 9) [ "deopt"() ]
; CHECK-NEXT: call void (i1, ...) @llvm.experimental.guard(i1 [[TMP3]], i32 9) [ "deopt"() ]
; CHECK-NEXT: [[I_NEXT]] = add nuw i32 [[I]], 1
; CHECK-NEXT: [[CONTINUE:%.*]] = icmp ne i32 [[I]], [[N]]
; CHECK-NEXT: br i1 [[CONTINUE]], label [[LOOP]], label [[EXIT:%.*]]
@ -1715,11 +1720,14 @@ define i32 @ne_latch_dom_check_preinc(i32* %array, i32 %length, i32 %n) {
; CHECK-NEXT: [[TMP5:%.*]] = icmp sle i32 [[N:%.*]], 0
; CHECK-NEXT: br i1 [[TMP5]], label [[EXIT:%.*]], label [[LOOP_PREHEADER:%.*]]
; CHECK: loop.preheader:
; CHECK-NEXT: [[TMP0:%.*]] = add i32 [[LENGTH:%.*]], -1
; CHECK-NEXT: [[TMP1:%.*]] = icmp ule i32 [[N]], [[TMP0]]
; CHECK-NEXT: [[TMP2:%.*]] = icmp ult i32 0, [[LENGTH]]
; CHECK-NEXT: [[TMP3:%.*]] = and i1 [[TMP2]], [[TMP1]]
; CHECK-NEXT: br label [[LOOP:%.*]]
; CHECK: loop:
; CHECK-NEXT: [[I:%.*]] = phi i32 [ [[I_NEXT:%.*]], [[LOOP]] ], [ 0, [[LOOP_PREHEADER]] ]
; CHECK-NEXT: [[WITHIN_BOUNDS:%.*]] = icmp ult i32 [[I]], [[LENGTH:%.*]]
; CHECK-NEXT: call void (i1, ...) @llvm.experimental.guard(i1 [[WITHIN_BOUNDS]], i32 9) [ "deopt"() ]
; CHECK-NEXT: call void (i1, ...) @llvm.experimental.guard(i1 [[TMP3]], i32 9) [ "deopt"() ]
; CHECK-NEXT: [[I_NEXT]] = add nuw i32 [[I]], 1
; CHECK-NEXT: [[CONTINUE:%.*]] = icmp ne i32 [[I]], [[N]]
; CHECK-NEXT: br i1 [[CONTINUE]], label [[LOOP]], label [[EXIT_LOOPEXIT:%.*]]