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[LoopVectorize] Address comments on PR llvm#107004 left post-commit
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* Rename Speculative -> Uncountable and update tests.
* Add comments explaining why it's safe to ignore the predicates
when building up a list of exiting blocks.
* Reshuffle some code to do (hopefully) cheaper checks first.
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@david-arm
david-arm committed Sep 23, 2024
1 parent 02ee96e commit 7dad4ad
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Showing 4 changed files with 54 additions and 52 deletions.
22 changes: 12 additions & 10 deletions llvm/include/llvm/Transforms/Vectorize/LoopVectorizationLegality.h
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Original file line number Diff line number Diff line change
Expand Up @@ -377,19 +377,19 @@ class LoopVectorizationLegality {
return LAI->getDepChecker().getMaxSafeVectorWidthInBits();
}

/// Returns true if the loop has a speculative early exit, i.e. an
/// Returns true if the loop has an uncountable early exit, i.e. an
/// uncountable exit that isn't the latch block.
bool hasSpeculativeEarlyExit() const { return HasSpeculativeEarlyExit; }
bool hasUncountableEarlyExit() const { return HasUncountableEarlyExit; }

/// Returns the speculative early exiting block.
BasicBlock *getSpeculativeEarlyExitingBlock() const {
/// Returns the uncountable early exiting block.
BasicBlock *getUncountableEarlyExitingBlock() const {
assert(getUncountableExitingBlocks().size() == 1 &&
"Expected only a single uncountable exiting block");
return getUncountableExitingBlocks()[0];
}

/// Returns the destination of a speculative early exiting block.
BasicBlock *getSpeculativeEarlyExitBlock() const {
/// Returns the destination of an uncountable early exiting block.
BasicBlock *getUncountableEarlyExitBlock() const {
assert(getUncountableExitBlocks().size() == 1 &&
"Expected only a single uncountable exit block");
return getUncountableExitBlocks()[0];
Expand Down Expand Up @@ -603,15 +603,17 @@ class LoopVectorizationLegality {
/// the use of those function variants.
bool VecCallVariantsFound = false;

/// Indicates whether this loop has a speculative early exit, i.e. an
/// Indicates whether this loop has an uncountable early exit, i.e. an
/// uncountable exiting block that is not the latch.
bool HasSpeculativeEarlyExit = false;
bool HasUncountableEarlyExit = false;

/// Keep track of all the loop exiting blocks.
/// Keep track of all the countable and uncountable exiting blocks if
/// the exact backedge taken count is not computable.
SmallVector<BasicBlock *, 4> CountableExitingBlocks;
SmallVector<BasicBlock *, 4> UncountableExitingBlocks;

/// Keep track of the destinations of all uncountable exits.
/// Keep track of the destinations of all uncountable exits if the
/// exact backedge taken count is not computable.
SmallVector<BasicBlock *, 4> UncountableExitBlocks;
};

Expand Down
67 changes: 36 additions & 31 deletions llvm/lib/Transforms/Vectorize/LoopVectorizationLegality.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1473,13 +1473,13 @@ bool LoopVectorizationLegality::isVectorizableEarlyExitLoop() {

// Keep a record of all the exiting blocks.
SmallVector<const SCEVPredicate *, 4> Predicates;
for (BasicBlock *BB1 : ExitingBlocks) {
for (BasicBlock *BB : ExitingBlocks) {
const SCEV *EC =
PSE.getSE()->getPredicatedExitCount(TheLoop, BB1, &Predicates);
PSE.getSE()->getPredicatedExitCount(TheLoop, BB, &Predicates);
if (isa<SCEVCouldNotCompute>(EC)) {
UncountableExitingBlocks.push_back(BB1);
UncountableExitingBlocks.push_back(BB);

SmallVector<BasicBlock *, 2> Succs(successors(BB1));
SmallVector<BasicBlock *, 2> Succs(successors(BB));
if (Succs.size() != 2) {
reportVectorizationFailure(
"Early exiting block does not have exactly two successors",
Expand All @@ -1488,17 +1488,21 @@ bool LoopVectorizationLegality::isVectorizableEarlyExitLoop() {
return false;
}

BasicBlock *BB2;
BasicBlock *ExitBlock;
if (!TheLoop->contains(Succs[0]))
BB2 = Succs[0];
ExitBlock = Succs[0];
else {
assert(!TheLoop->contains(Succs[1]));
BB2 = Succs[1];
ExitBlock = Succs[1];
}
UncountableExitBlocks.push_back(BB2);
UncountableExitBlocks.push_back(ExitBlock);
} else
CountableExitingBlocks.push_back(BB1);
CountableExitingBlocks.push_back(BB);
}
// We can safely ignore the predicates here because when vectorizing the loop
// the PredicatatedScalarEvolution class will keep track of all predicates
// for each exiting block anyway. This happens when calling
// PSE.getSymbolicMaxBackedgeTakenCount() below.
Predicates.clear();

// We only support one uncountable early exit.
Expand All @@ -1513,13 +1517,25 @@ bool LoopVectorizationLegality::isVectorizableEarlyExitLoop() {
// The only supported early exit loops so far are ones where the early
// exiting block is a unique predecessor of the latch block.
BasicBlock *LatchPredBB = LatchBB->getUniquePredecessor();
if (LatchPredBB != getSpeculativeEarlyExitingBlock()) {
if (LatchPredBB != getUncountableEarlyExitingBlock()) {
reportVectorizationFailure("Early exit is not the latch predecessor",
"Cannot vectorize early exit loop",
"EarlyExitNotLatchPredecessor", ORE, TheLoop);
return false;
}

// The latch block must have a countable exit.
if (isa<SCEVCouldNotCompute>(
PSE.getSE()->getPredicatedExitCount(TheLoop, LatchBB, &Predicates))) {
reportVectorizationFailure(
"Cannot determine exact exit count for latch block",
"Cannot vectorize early exit loop",
"UnknownLatchExitCountEarlyExitLoop", ORE, TheLoop);
return false;
}
assert(llvm::is_contained(CountableExitingBlocks, LatchBB) &&
"Latch block not found in list of countable exits!");

// Check to see if there are instructions that could potentially generate
// exceptions or have side-effects.
auto IsSafeOperation = [](Instruction *I) -> bool {
Expand Down Expand Up @@ -1555,18 +1571,8 @@ bool LoopVectorizationLegality::isVectorizableEarlyExitLoop() {
}
}

// The latch block must have a countable exit.
if (isa<SCEVCouldNotCompute>(
PSE.getSE()->getPredicatedExitCount(TheLoop, LatchBB, &Predicates))) {
reportVectorizationFailure(
"Cannot determine exact exit count for latch block",
"Cannot vectorize early exit loop",
"UnknownLatchExitCountEarlyExitLoop", ORE, TheLoop);
return false;
}

// The vectoriser cannot handle loads that occur after the early exit block.
assert(LatchBB->getUniquePredecessor() == getSpeculativeEarlyExitingBlock() &&
assert(LatchBB->getUniquePredecessor() == getUncountableEarlyExitingBlock() &&
"Expected latch predecessor to be the early exiting block");

// TODO: Handle loops that may fault.
Expand All @@ -1580,16 +1586,15 @@ bool LoopVectorizationLegality::isVectorizableEarlyExitLoop() {
return false;
}

LLVM_DEBUG(
dbgs()
<< "LV: Found an early exit. Retrying with speculative exit count.\n");
[[maybe_unused]] const SCEV *SpecExitCount =
[[maybe_unused]] const SCEV *SymbolicMaxBTC =
PSE.getSymbolicMaxBackedgeTakenCount();
assert(!isa<SCEVCouldNotCompute>(SpecExitCount) &&
// Since we have an exact exit count for the latch and the early exit
// dominates the latch, then this should guarantee a computed SCEV value.
assert(!isa<SCEVCouldNotCompute>(SymbolicMaxBTC) &&
"Failed to get symbolic expression for backedge taken count");

LLVM_DEBUG(dbgs() << "LV: Found speculative backedge taken count: "
<< *SpecExitCount << '\n');
LLVM_DEBUG(dbgs() << "LV: Found an early exit loop with symbolic max "
"backedge taken count: "
<< *SymbolicMaxBTC << '\n');
return true;
}

Expand Down Expand Up @@ -1653,15 +1658,15 @@ bool LoopVectorizationLegality::canVectorize(bool UseVPlanNativePath) {
return false;
}

HasSpeculativeEarlyExit = false;
HasUncountableEarlyExit = false;
if (isa<SCEVCouldNotCompute>(PSE.getBackedgeTakenCount())) {
if (!isVectorizableEarlyExitLoop()) {
if (DoExtraAnalysis)
Result = false;
else
return false;
} else
HasSpeculativeEarlyExit = true;
HasUncountableEarlyExit = true;
}

// Go over each instruction and look at memory deps.
Expand Down
2 changes: 1 addition & 1 deletion llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -9792,7 +9792,7 @@ bool LoopVectorizePass::processLoop(Loop *L) {
return false;
}

if (LVL.hasSpeculativeEarlyExit()) {
if (LVL.hasUncountableEarlyExit()) {
reportVectorizationFailure(
"Auto-vectorization of early exit loops is not yet supported.",
"Auto-vectorization of early exit loops is not yet supported.",
Expand Down
15 changes: 5 additions & 10 deletions llvm/test/Transforms/LoopVectorize/simple_early_exit.ll
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Original file line number Diff line number Diff line change
Expand Up @@ -7,8 +7,7 @@ declare void @init_mem(ptr, i64);

define i64 @same_exit_block_pre_inc_use1() {
; DEBUG-LABEL: LV: Checking a loop in 'same_exit_block_pre_inc_use1'
; DEBUG: LV: Found an early exit. Retrying with speculative exit count.
; DEBUG-NEXT: LV: Found speculative backedge taken count: 63
; DEBUG: LV: Found an early exit loop with symbolic max backedge taken count: 63
; DEBUG-NEXT: LV: We can vectorize this loop!
; DEBUG-NEXT: LV: Not vectorizing: Auto-vectorization of early exit loops is not yet supported.
; CHECK-LABEL: define i64 @same_exit_block_pre_inc_use1() {
Expand Down Expand Up @@ -1089,8 +1088,7 @@ loop.end:

define i64 @loop_contains_safe_call() {
; DEBUG-LABEL: LV: Checking a loop in 'loop_contains_safe_call'
; DEBUG: LV: Found an early exit. Retrying with speculative exit count.
; DEBUG-NEXT: LV: Found speculative backedge taken count: 63
; DEBUG: LV: Found an early exit loop with symbolic max backedge taken count: 63
; DEBUG-NEXT: LV: We can vectorize this loop!
; CHECK-LABEL: define i64 @loop_contains_safe_call() {
; CHECK-NEXT: entry:
Expand Down Expand Up @@ -1193,8 +1191,7 @@ loop.end:

define i64 @loop_contains_safe_div() {
; DEBUG-LABEL: LV: Checking a loop in 'loop_contains_safe_div'
; DEBUG: LV: Found an early exit. Retrying with speculative exit count.
; DEBUG-NEXT: LV: Found speculative backedge taken count: 63
; DEBUG: LV: Found an early exit loop with symbolic max backedge taken count: 63
; DEBUG-NEXT: LV: We can vectorize this loop!
; CHECK-LABEL: define i64 @loop_contains_safe_div() {
; CHECK-NEXT: entry:
Expand Down Expand Up @@ -1347,8 +1344,7 @@ loop.end:

define i64 @loop_contains_load_after_early_exit(ptr dereferenceable(1024) align(8) %p2) {
; DEBUG-LABEL: LV: Checking a loop in 'loop_contains_load_after_early_exit'
; DEBUG: LV: Found an early exit. Retrying with speculative exit count.
; DEBUG-NEXT: LV: Found speculative backedge taken count: 63
; DEBUG: LV: Found an early exit loop with symbolic max backedge taken count: 63
; DEBUG-NEXT: LV: We can vectorize this loop!
; DEBUG-NEXT: LV: Not vectorizing: Auto-vectorization of early exit loops is not yet supported.
; CHECK-LABEL: define i64 @loop_contains_load_after_early_exit(
Expand Down Expand Up @@ -1695,8 +1691,7 @@ declare void @abort()
; early is loop invariant.
define i32 @diff_blocks_invariant_early_exit_cond(ptr %s) {
; DEBUG-LABEL: LV: Checking a loop in 'diff_blocks_invariant_early_exit_cond'
; DEBUG: LV: Found an early exit. Retrying with speculative exit count.
; DEBUG-NEXT: LV: Found speculative backedge taken count: 275
; DEBUG: LV: Found an early exit loop with symbolic max backedge taken count: 275
; DEBUG: LV: Not vectorizing: Auto-vectorization of early exit loops is not yet supported.
; CHECK-LABEL: define i32 @diff_blocks_invariant_early_exit_cond(
; CHECK-SAME: ptr [[S:%.*]]) {
Expand Down

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