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Reland [compiler] Improve code generated for patterns like "x >> 1 == 0"

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This is a reland of 2dc4037049

Original change's description:
> [compiler] Improve code generated for patterns like "x >> 1 == 0"
>
> Change-Id: I79575ba61a3bdea93468f48d66a3cb3edd0e1442
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3506504
> Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
> Commit-Queue: Darius Mercadier <dmercadier@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#79419}

Change-Id: Iad111f8d4bb40a295903dd67f66c8ecd9c4eadd9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3514072
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Darius Mercadier <dmercadier@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79443}
This commit is contained in:
Darius M 2022-03-10 13:11:15 +01:00 committed by V8 LUCI CQ
parent cf25b3bc53
commit dca3b16e90
1 changed files with 64 additions and 2 deletions
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66
src/compiler/machine-operator-reducer.cc
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@ -1348,13 +1348,33 @@ Reduction MachineOperatorReducer::ReduceProjection(size_t index, Node* node) {
return NoChange();
}
namespace {
// Returns true if "value << shift >> shift == value". This can be interpreted
// as "left shifting |value| by |shift| doesn't shift away significant bits".
// Or, equivalently, "left shifting |value| by |shift| doesn't have signed
// overflow".
bool CanRevertLeftShiftWithRightShift(int32_t value, int32_t shift) {
if (shift < 0 || shift >= 32) {
// This shift would be UB in C++
return false;
}
if (static_cast<int32_t>(static_cast<uint32_t>(value) << shift) >> shift !=
static_cast<int32_t>(value)) {
return false;
}
return true;
}
} // namespace
Reduction MachineOperatorReducer::ReduceWord32Comparisons(Node* node) {
DCHECK(node->opcode() == IrOpcode::kInt32LessThan ||
node->opcode() == IrOpcode::kInt32LessThanOrEqual ||
node->opcode() == IrOpcode::kUint32LessThan ||
node->opcode() == IrOpcode::kUint32LessThanOrEqual);
Int32BinopMatcher m(node);
// (x >>> K) < (y >>> K) => x < y if only zeros shifted out
// (x >> K) < (y >> K) => x < y if only zeros shifted out
if (m.left().op() == machine()->Word32SarShiftOutZeros() &&
m.right().op() == machine()->Word32SarShiftOutZeros()) {
Int32BinopMatcher mleft(m.left().node());
@ -1366,6 +1386,36 @@ Reduction MachineOperatorReducer::ReduceWord32Comparisons(Node* node) {
return Changed(node);
}
}
// Simplifying (x >> n) <= k into x <= (k << n), with "k << n" being
// computed here at compile time.
if (m.right().HasResolvedValue() &&
m.left().op() == machine()->Word32SarShiftOutZeros()) {
uint32_t right = m.right().ResolvedValue();
Int32BinopMatcher mleft(m.left().node());
if (mleft.right().HasResolvedValue()) {
auto shift = mleft.right().ResolvedValue();
if (CanRevertLeftShiftWithRightShift(right, shift)) {
node->ReplaceInput(0, mleft.left().node());
node->ReplaceInput(1, Int32Constant(right << shift));
return Changed(node);
}
}
}
// Simplifying k <= (x >> n) into (k << n) <= x, with "k << n" being
// computed here at compile time.
if (m.left().HasResolvedValue() &&
m.right().op() == machine()->Word32SarShiftOutZeros()) {
uint32_t left = m.left().ResolvedValue();
Int32BinopMatcher mright(m.right().node());
if (mright.right().HasResolvedValue()) {
auto shift = mright.right().ResolvedValue();
if (CanRevertLeftShiftWithRightShift(left, shift)) {
node->ReplaceInput(0, Int32Constant(left << shift));
node->ReplaceInput(1, mright.left().node());
return Changed(node);
}
}
}
return NoChange();
}
@ -1405,7 +1455,7 @@ Reduction MachineOperatorReducer::ReduceWord64Comparisons(Node* node) {
return Changed(node).FollowedBy(Reduce(node));
}
// (x >>> K) < (y >>> K) => x < y if only zeros shifted out
// (x >> K) < (y >> K) => x < y if only zeros shifted out
// This is useful for Smi untagging, which results in such a shift.
if (m.left().op() == machine()->Word64SarShiftOutZeros() &&
m.right().op() == machine()->Word64SarShiftOutZeros()) {
@ -2202,6 +2252,18 @@ MachineOperatorReducer::ReduceWord32EqualForConstantRhs(Node* lhs,
}
}
}
// Replaces (x >> n) == k with x == k << n, with "k << n" being computed
// here at compile time.
if (lhs->op() == machine()->Word32SarShiftOutZeros() &&
lhs->UseCount() == 1) {
typename WordNAdapter::UintNBinopMatcher mshift(lhs);
if (mshift.right().HasResolvedValue()) {
int32_t shift = static_cast<int32_t>(mshift.right().ResolvedValue());
if (CanRevertLeftShiftWithRightShift(rhs, shift)) {
return std::make_pair(mshift.left().node(), rhs << shift);
}
}
}
return {};
}