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Bring glue RVO in line with canElideCopy() #22199

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Bring glue RVO in line with canElideCopy() #22199

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00f1230
Bring glue RVO in line with `canElideCopy()`
limepoutine Dec 6, 2025
0220b3a
Fix NRVO incompatibility with contracts
limepoutine Dec 7, 2025
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4 changes: 4 additions & 0 deletions compiler/src/dmd/expressionsem.d
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Original file line number Diff line number Diff line change
Expand Up @@ -674,6 +674,10 @@ bool isLvalue(Expression _this)
* Determine if copy elision is allowed when copying an expression to
* a typed storage. This basically elides a restricted subset of so-called
* "pure" rvalues, i.e. expressions with no reference semantics.
*
* Note: Please try to keep `dmd.glue.e2ir.toElem()` in sync with this.
* It is not destructive to fail to elide a copy, but it is always better
* to stay consistent.
*/
bool canElideCopy(Expression e, Type to, bool checkMod = true)
{
Expand Down
98 changes: 91 additions & 7 deletions compiler/src/dmd/glue/e2ir.d
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Expand Up @@ -692,7 +692,7 @@ elem* toElem(Expression e, ref IRState irs, elem* ehidden = null)
}
}

Symbol* s = toSymbol(se.var);
Symbol* s = toSymbolNRVO(se.var);

// VarExp generated for `__traits(initSymbol, Aggregate)`?
if (auto symDec = se.var.isSymbolDeclaration())
Expand All @@ -718,9 +718,7 @@ elem* toElem(Expression e, ref IRState irs, elem* ehidden = null)
if (se.var.toParent2())
fd = se.var.toParent2().isFuncDeclaration();

const bool nrvo = fd && (fd.isNRVO && fd.nrvo_var == se.var || se.var.nrvo && fd.shidden);
if (nrvo)
s = cast(Symbol*)fd.shidden;
const bool nrvo = fd && s == fd.shidden;

if (s.Sclass == SC.auto_ || s.Sclass == SC.parameter || s.Sclass == SC.shadowreg)
{
Expand Down Expand Up @@ -3185,15 +3183,17 @@ elem* toElem(Expression e, ref IRState irs, elem* ehidden = null)
auto ctfecond = ce.econd.op == EXP.not ? (cast(NotExp)ce.econd).e1 : ce.econd;
if (auto ve = ctfecond.isVarExp())
if (ve.var && ve.var.ident == Id.ctfe)
return toElem(ctfecond is ce.econd ? ce.e2 : ce.e1, irs);
return toElem(ctfecond is ce.econd ? ce.e2 : ce.e1, irs, ehidden);

elem* ec = toElem(ce.econd, irs);

elem* eleft = toElem(ce.e1, irs);
elem* eleft = toElem(ce.e1, irs, ehidden);
if (irs.params.cov && ce.e1.loc.linnum)
eleft = el_combine(incUsageElem(irs, ce.e1.loc), eleft);

elem* eright = toElem(ce.e2, irs);
if (ehidden)
ehidden = el_copytree(ehidden);
elem* eright = toElem(ce.e2, irs, ehidden);
if (irs.params.cov && ce.e2.loc.linnum)
eright = el_combine(incUsageElem(irs, ce.e2.loc), eright);

Expand Down Expand Up @@ -4226,6 +4226,90 @@ elem* toElem(Expression e, ref IRState irs, elem* ehidden = null)
return el_ptr(toSymbol(e));
}

/*****************************************************/
/* RVO special cases */
/*****************************************************/

elem* moveToHiddenPtr(Expression e)
{
elem* el = toElem(e, irs);
elem* ea = elAssign(el_una(OPind, el.Ety, ehidden), el, e.type, null);
elem* eh = el_una(OPind, el.Ety, el_copytree(ehidden));
eh.ET = el.ET;
return el_combine(ea, eh);
}

elem* doCallRVO(CallExp e)
{
FuncDeclaration fd;

if (auto dve = e.e1.isDotVarExp())
{
fd = dve.var.isFuncDeclaration();
auto sle = dve.e1.isStructLiteralExp();
if (fd && fd.isCtorDeclaration() && sle)
{
assert(ehidden.Eoper == OPvar);
sle.sym = ehidden.Vsym;
return toElem(e, irs);
}
}
else
{
fd = e.f;
}

// If fd is eligible for RVO, invoke visitCall directly to pick up ehidden
Type t = e.e1.type.toBasetype();
if (t.ty == Tdelegate)
t = t.nextOf();
if (t.ty == Tfunction && retStyle(cast(TypeFunction)t, fd && fd.needThis()) == RET.stack)
return visitCall(e);

// The return value is passed in registers
return moveToHiddenPtr(e);
}

elem* doVariableRVO(VarExp e)
{
// Replace e with ehidden if e.var is already the hidden variable
if (ehidden.Eoper == OPvar && ehidden.Vsym == toSymbolNRVO(e.var))
{
elem* el = toElem(e, irs);
elem* eh = el_una(OPind, el.Ety, ehidden);
eh.ET = el.ET;
el_free(el);
return eh;
}

// Otherwise a move is inevitable
return moveToHiddenPtr(e);
}

elem* doStructLiteralRVO(StructLiteralExp e)
{
// Patch e.sym and proceed as if there is no RVO
assert(ehidden.Eoper == OPvar);
e.sym = ehidden.Vsym;
return toElem(e, irs);
}

if (ehidden)
{
// Catch unhandled RVO cases
// Please keep in sync with dmd.expressionsem.canElideCopy()
switch (e.op)
{
case EXP.comma: return visitComma(e.isCommaExp());
case EXP.question: return visitCond(e.isCondExp());
case EXP.call: return doCallRVO(e.isCallExp());
case EXP.variable: return doVariableRVO(e.isVarExp());
case EXP.dotVariable: return moveToHiddenPtr(e);
case EXP.structLiteral: return doStructLiteralRVO(e.isStructLiteralExp());
default: assert(0);
}
}

switch (e.op)
{
default: return visit(e);
Expand Down
146 changes: 52 additions & 94 deletions compiler/src/dmd/glue/s2ir.d
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Original file line number Diff line number Diff line change
Expand Up @@ -38,7 +38,7 @@ import dmd.dsymbol;
import dmd.dstruct;
import dmd.dtemplate;
import dmd.expression;
import dmd.expressionsem : getDsymbol, toInteger;
import dmd.expressionsem : canElideCopy, getDsymbol, toInteger;
import dmd.func;
import dmd.id;
import dmd.init;
Expand Down Expand Up @@ -538,143 +538,101 @@ void Statement_toIR(Statement s, ref IRState irs, StmtState* stmtstate)
void visitReturn(ReturnStatement s)
{
//printf("s2ir.ReturnStatement: %s\n", toChars(s.exp));
BlockState* blx = irs.blx;
BC bc;

incUsage(irs, s.loc);
void finish()
void finish(elem* e = null)
{
block* finallyBlock;
if (config.ehmethod != EHmethod.EH_DWARF &&
!irs.isNothrow() &&
(finallyBlock = stmtstate.getFinallyBlock()) != null)
BC bc = BC.ret;
BlockState* blx = irs.blx;

if (e)
{
assert(finallyBlock.bc == BC._finally);
blx.curblock.appendSucc(finallyBlock);
elem_setLoc(e, s.loc);
block_appendexp(blx.curblock, e);
bc = BC.retexp;
}

if (config.ehmethod != EHmethod.EH_DWARF &&!irs.isNothrow())
{
if (block* finallyBlock = stmtstate.getFinallyBlock())
{
assert(finallyBlock.bc == BC._finally);
blx.curblock.appendSucc(finallyBlock);
}
}

block_setLoc(blx.curblock, s.loc);
block_next(blx, bc, null);
}

incUsage(irs, s.loc);

if (!s.exp)
{
bc = BC.ret;
return finish();
}

elem* e;

FuncDeclaration func = irs.getFunc();
assert(func);
auto tf = func.type.isTypeFunction();
assert(tf);

RET retmethod = retStyle(tf, func.needThis());
if (retmethod == RET.stack)
if (retStyle(tf, func.needThis()) == RET.stack)
{
elem* es;
bool writetohp;
bool nrvo = func.isNRVO && func.nrvo_var;
bool urvo = !nrvo && canElideCopy(s.exp, s.exp.type, false);

/* If returning struct literal, write result
* directly into return value
*/
if (auto sle = s.exp.isStructLiteralExp())
{
sle.sym = irs.shidden;
writetohp = true;
}
/* Detect function call that returns the same struct
* and construct directly into *shidden
*/
else if (auto ce = s.exp.isCallExp())
{
if (ce.e1.op == EXP.variable || ce.e1.op == EXP.star)
{
Type t = ce.e1.type.toBasetype();
if (t.ty == Tdelegate)
t = t.nextOf();
if (t.ty == Tfunction && retStyle(cast(TypeFunction)t, ce.f && ce.f.needThis()) == RET.stack)
{
e = toElemDtor(s.exp, irs, el_var(irs.shidden));
e = el_una(OPaddr, TYnptr, e);
goto L1;
}
}
else if (auto dve = ce.e1.isDotVarExp())
{
auto fd = dve.var.isFuncDeclaration();
if (fd && fd.isCtorDeclaration())
{
if (auto sle = dve.e1.isStructLiteralExp())
{
sle.sym = irs.shidden;
writetohp = true;
}
}
Type t = ce.e1.type.toBasetype();
if (t.ty == Tdelegate)
t = t.nextOf();
if (t.ty == Tfunction && retStyle(cast(TypeFunction)t, fd && fd.needThis()) == RET.stack)
{
e = toElemDtor(s.exp, irs, el_var(irs.shidden));
e = el_una(OPaddr, TYnptr, e);
goto L1;
}
}
}
e = toElemDtor(s.exp, irs);
// Pass shidden in ehidden for URVO
elem* ehidden = urvo ? el_var(irs.shidden) : null;
elem* e = toElemDtor(s.exp, irs, ehidden);
assert(e);

if (writetohp ||
(func.isNRVO && func.nrvo_var))
if (urvo)
{
// In URVO cases, toElemDtor already ensures e references
// the hidden pointer, so there is no need to rewrite
e = el_una(OPaddr, TYnptr, e);
}
else if (nrvo)
{
// Return value via hidden pointer passed as parameter
// Write exp; return shidden;
es = e;
// Rewrite to (exp, shidden)
e = el_combine(e, el_var(irs.shidden));
}
else
{
// Return value via hidden pointer passed as parameter
// Write *shidden=exp; return shidden;
es = el_una(OPind,e.Ety,el_var(irs.shidden));
// Rewrite to (*shidden=exp, shidden)
elem* es = el_una(OPind, e.Ety, el_var(irs.shidden));
es = elAssign(es, e, s.exp.type, null);
e = el_combine(es, el_var(irs.shidden));
}
e = el_var(irs.shidden);
e = el_bin(OPcomma, e.Ety, es, e);

return finish(e);
}
else if (tf.isRef)
{
// Reference return, so convert to a pointer
e = toElemDtor(s.exp, irs);

elem* e = toElemDtor(s.exp, irs);
assert(e);

if (tf.isRef)
{
/* already taken care of for vresult in buildResultVar() and semantic3.d
* https://issues.dlang.org/show_bug.cgi?id=19384
*/
if (func.vresult)
{
if (BlitExp be = s.exp.isBlitExp())
{
if (VarExp ve = be.e1.isVarExp())
{
if (ve.var == func.vresult)
goto Lskip;
return finish(e);
}
}
}

// Reference return, so convert to a pointer
e = addressElem(e, s.exp.type.pointerTo());
Lskip:
}
else
{
e = toElemDtor(s.exp, irs);
assert(e);
}
L1:
elem_setLoc(e, s.loc);
block_appendexp(blx.curblock, e);
bc = BC.retexp;
// if (type_zeroCopy(Type_toCtype(s.exp.type)))
// bc = BC.ret;
finish();

finish(e);
}

/**************************************
Expand Down
22 changes: 22 additions & 0 deletions compiler/src/dmd/glue/tocsym.d
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Expand Up @@ -103,6 +103,7 @@ Symbol* toSymbolX(Dsymbol ds, const(char)* prefix, SC sclass, type* t, const(cha
}

/*************************************
* Convert D symbol to backend symbol.
*/

Symbol* toSymbol(Dsymbol s)
Expand Down Expand Up @@ -550,6 +551,27 @@ Symbol* toSymbol(Dsymbol s)
return v.result;
}

/*************************************
* Convert D symbol to backend symbol.
* Unlike toSymbol, this function also resolves
* NRVO variables to the hidden pointer.
*/
Symbol* toSymbolNRVO(Dsymbol s)
{
if (auto parent = s.toParent2())
{
auto fd = parent.isFuncDeclaration();
auto var = s.isVarDeclaration();
bool nrvo = fd && var && (fd.isNRVO && fd.nrvo_var == var || var.nrvo && fd.shidden);

if (nrvo)
{
return cast(Symbol*)fd.shidden;
}
}

return toSymbol(s);
}

/*************************************
* Create Windows import symbol from backend Symbol.
Expand Down
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