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Internals: Do not emit temporaries for atomic assignments. (#6217)

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Added test for a particularly convoluted case requiring fixup in
V3Premit. To help with statistics stability, also prevent V3Premit from
introducing temporaries for assignment where the RHS reads the LHS, but
the assignment is known to be atomic (by emitted C++ semantics).

Also rename `createWideTemp` to `createTemp`, as it is used for non-wide
expressions as well.
This commit is contained in:
Geza Lore
2025-07-23 11:48:55 +02:00
committed by GitHub
parent 344fabf56a
commit 2958a5aaae
4 changed files with 74 additions and 9 deletions
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src/V3Premit.cpp
+14 -9
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@@ -46,6 +46,7 @@ class PremitVisitor final : public VNVisitor {
// STATE - across all visitors
VDouble0 m_extractedToConstPool; // Statistic tracking
VDouble0 m_temporaryVarsCreated; // Statistic tracking
// STATE - for current visit position (use VL_RESTORER)
AstCFunc* m_cfuncp = nullptr; // Current block
@@ -62,12 +63,12 @@ class PremitVisitor final : public VNVisitor {
if (!nodep->isWide()) return; // Not wide
if (m_assignLhs) return; // This is an lvalue!
UASSERT_OBJ(!VN_IS(nodep->firstAbovep(), ArraySel), nodep, "Should have been ignored");
createWideTemp(nodep);
createTemp(nodep);
}
AstVar* createWideTemp(AstNodeExpr* nodep) {
AstVar* createTemp(AstNodeExpr* nodep) {
UASSERT_OBJ(m_stmtp, nodep, "Attempting to create temporary with no insertion point");
UINFO(4, "createWideTemp: " << nodep);
UINFO(4, "createTemp: " << nodep);
VNRelinker relinker;
nodep->unlinkFrBack(&relinker);
@@ -93,6 +94,7 @@ class PremitVisitor final : public VNVisitor {
const std::string name = "__Vtemp_" + std::to_string(++m_tmpVarCnt);
varp = new AstVar{flp, VVarType::STMTTEMP, name, nodep->dtypep()};
m_cfuncp->addInitsp(varp);
++m_temporaryVarsCreated;
// Put assignment before the referencing statement
assignp = new AstAssign{flp, new AstVarRef{flp, varp, VAccess::WRITE}, nodep};
@@ -221,9 +223,10 @@ class PremitVisitor final : public VNVisitor {
}
}
if (rhsReadsLhs(nodep)) {
// Need to do this even if not wide, as e.g. a select may be on a wide operator
createWideTemp(nodep->rhsp());
// If the RHS reads the LHS, we need a temporary unless the update is atomic
const bool isAtomic = VN_IS(nodep->lhsp(), VarRef) && !nodep->lhsp()->isWide();
if (!isAtomic && rhsReadsLhs(nodep)) {
createTemp(nodep->rhsp());
} else {
iterateAndNextNull(nodep->rhsp());
}
@@ -290,7 +293,7 @@ class PremitVisitor final : public VNVisitor {
void visit(AstCvtPackedToArray* nodep) override {
iterateChildren(nodep);
checkNode(nodep);
if (!VN_IS(nodep->backp(), NodeAssign)) createWideTemp(nodep);
if (!VN_IS(nodep->backp(), NodeAssign)) createTemp(nodep);
}
void visit(AstCvtUnpackedToQueue* nodep) override {
iterateChildren(nodep);
@@ -330,7 +333,7 @@ class PremitVisitor final : public VNVisitor {
&& !VN_IS(nodep->condp(), VarRef)) {
// We're going to need the expression several times in the expanded code,
// so might as well make it a common expression
createWideTemp(nodep->condp());
createTemp(nodep->condp());
VIsCached::clearCacheTree();
}
checkNode(nodep);
@@ -342,7 +345,7 @@ class PremitVisitor final : public VNVisitor {
for (AstNodeExpr *expp = nodep->exprsp(), *nextp; expp; expp = nextp) {
nextp = VN_AS(expp->nextp(), NodeExpr);
if (expp->isString() && !VN_IS(expp, VarRef)) {
AstVar* const varp = createWideTemp(expp);
AstVar* const varp = createTemp(expp);
// Do not remove VarRefs to this in V3Const
varp->noSubst(true);
}
@@ -360,6 +363,8 @@ public:
~PremitVisitor() override {
V3Stats::addStat("Optimizations, Prelim extracted value to ConstPool",
m_extractedToConstPool);
V3Stats::addStat("Optimizations, Prelim temporary variables created",
m_temporaryVarsCreated);
}
};