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codegen: add handling for undefined phinode values
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The optimization pass often uses values for phi values (and thus by
extension, also for pi, phic and upsilon values) that are invalid. We
make sure that these have a null pointer, so that we can detect that
case at runtime (at the cost of slightly worse code generation for
them), but it means we need to be very careful to check for that.

This is identical to #39747, which added the equivalent code to the
other side of the conditional there, but missed some additional
relevant, but rare, cases that are observed to be possible.

The `emit_isa_and_defined` is derived from the LLVM name for this
operation: `isa_and_nonnull<T>`.

Secondly, we also optimize `emit_unionmove` to change a bad IR case to a
better IR form.

Fix #44501
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@vtjnash
vtjnash committed May 9, 2022
1 parent 97c7009 commit 97ffa7f
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Showing 3 changed files with 83 additions and 70 deletions.
120 changes: 63 additions & 57 deletions src/cgutils.cpp
View file
Original file line number Diff line number Diff line change
Expand Up @@ -847,7 +847,7 @@ static bool is_uniontype_allunboxed(jl_value_t *typ)
return for_each_uniontype_small([&](unsigned, jl_datatype_t*) {}, typ, counter);
}

static Value *emit_typeof_boxed(jl_codectx_t &ctx, const jl_cgval_t &p);
static Value *emit_typeof_boxed(jl_codectx_t &ctx, const jl_cgval_t &p, bool maybenull=false);

static unsigned get_box_tindex(jl_datatype_t *jt, jl_value_t *ut)
{
Expand Down Expand Up @@ -902,16 +902,9 @@ static LoadInst *emit_nthptr_recast(jl_codectx_t &ctx, Value *v, ssize_t n, MDNo
}

static Value *boxed(jl_codectx_t &ctx, const jl_cgval_t &v);
static Value *emit_typeof(jl_codectx_t &ctx, Value *v, bool maybenull);

// Returns ctx.types().T_prjlvalue
static Value *emit_typeof(jl_codectx_t &ctx, Value *tt)
{
++EmittedTypeof;
assert(tt != NULL && !isa<AllocaInst>(tt) && "expected a conditionally boxed value");
return ctx.builder.CreateCall(prepare_call(jl_typeof_func), {tt});
}

static jl_cgval_t emit_typeof(jl_codectx_t &ctx, const jl_cgval_t &p)
static jl_cgval_t emit_typeof(jl_codectx_t &ctx, const jl_cgval_t &p, bool maybenull)
{
// given p, compute its type
if (p.constant)
Expand All @@ -924,7 +917,7 @@ static jl_cgval_t emit_typeof(jl_codectx_t &ctx, const jl_cgval_t &p)
return mark_julia_const(ctx, jl_typeof(tp));
}
}
return mark_julia_type(ctx, emit_typeof(ctx, p.V), true, jl_datatype_type);
return mark_julia_type(ctx, emit_typeof(ctx, p.V, maybenull), true, jl_datatype_type);
}
if (p.TIndex) {
Value *tindex = ctx.builder.CreateAnd(p.TIndex, ConstantInt::get(getInt8Ty(ctx.builder.getContext()), 0x7f));
Expand Down Expand Up @@ -959,7 +952,7 @@ static jl_cgval_t emit_typeof(jl_codectx_t &ctx, const jl_cgval_t &p)
BasicBlock *mergeBB = BasicBlock::Create(ctx.builder.getContext(), "merge", ctx.f);
ctx.builder.CreateCondBr(isnull, boxBB, unboxBB);
ctx.builder.SetInsertPoint(boxBB);
auto boxTy = emit_typeof(ctx, p.Vboxed);
auto boxTy = emit_typeof(ctx, p.Vboxed, maybenull);
ctx.builder.CreateBr(mergeBB);
boxBB = ctx.builder.GetInsertBlock(); // could have changed
ctx.builder.SetInsertPoint(unboxBB);
Expand All @@ -981,9 +974,9 @@ static jl_cgval_t emit_typeof(jl_codectx_t &ctx, const jl_cgval_t &p)
}

// Returns ctx.types().T_prjlvalue
static Value *emit_typeof_boxed(jl_codectx_t &ctx, const jl_cgval_t &p)
static Value *emit_typeof_boxed(jl_codectx_t &ctx, const jl_cgval_t &p, bool maybenull)
{
return boxed(ctx, emit_typeof(ctx, p));
return boxed(ctx, emit_typeof(ctx, p, maybenull));
}

static Value *emit_datatype_types(jl_codectx_t &ctx, Value *dt)
Expand Down Expand Up @@ -1224,6 +1217,24 @@ static Value *emit_nullcheck_guard2(jl_codectx_t &ctx, Value *nullcheck1,
});
}

// Returns typeof(v), or null if v is a null pointer at run time and maybenull is true.
// This is used when the value might have come from an undefined value (a PhiNode),
// yet we try to read its type to compute a union index when moving the value (a PiNode).
// Returns a ctx.types().T_prjlvalue typed Value
static Value *emit_typeof(jl_codectx_t &ctx, Value *v, bool maybenull)
{
++EmittedTypeof;
assert(v != NULL && !isa<AllocaInst>(v) && "expected a conditionally boxed value");
Function *typeof = prepare_call(jl_typeof_func);
if (maybenull)
return emit_guarded_test(ctx, null_pointer_cmp(ctx, v), Constant::getNullValue(typeof->getReturnType()), [&] {
// e.g. emit_typeof(ctx, v)
return ctx.builder.CreateCall(typeof, {v});
});
return ctx.builder.CreateCall(typeof, {v});
}


static void emit_type_error(jl_codectx_t &ctx, const jl_cgval_t &x, Value *type, const std::string &msg)
{
Value *msg_val = stringConstPtr(ctx.emission_context, ctx.builder, msg);
Expand Down Expand Up @@ -1353,7 +1364,7 @@ static std::pair<Value*, bool> emit_isa(jl_codectx_t &ctx, const jl_cgval_t &x,
BasicBlock *postBB = BasicBlock::Create(ctx.builder.getContext(), "post_isa", ctx.f);
ctx.builder.CreateCondBr(isboxed, isaBB, postBB);
ctx.builder.SetInsertPoint(isaBB);
Value *istype_boxed = ctx.builder.CreateICmpEQ(emit_typeof(ctx, x.Vboxed),
Value *istype_boxed = ctx.builder.CreateICmpEQ(emit_typeof(ctx, x.Vboxed, false),
track_pjlvalue(ctx, literal_pointer_val(ctx, intersected_type)));
ctx.builder.CreateBr(postBB);
isaBB = ctx.builder.GetInsertBlock(); // could have changed
Expand Down Expand Up @@ -1409,6 +1420,20 @@ static std::pair<Value*, bool> emit_isa(jl_codectx_t &ctx, const jl_cgval_t &x,
ConstantInt::get(getInt32Ty(ctx.builder.getContext()), 0)), false);
}

// If this might have been sourced from a PhiNode object, it is possible our
// Vboxed pointer itself is null (undef) at runtime even if we thought we should
// know exactly the type of the bytes that should have been inside.
//
// n.b. It is also possible the value is a ghost of some sort, and we will
// declare that the pointer is legal (for zero bytes) even though it might be undef.
static Value *emit_isa_and_defined(jl_codectx_t &ctx, const jl_cgval_t &val, jl_value_t *typ)
{
return emit_nullcheck_guard(ctx, val.ispointer() ? val.V : nullptr, [&] {
return emit_isa(ctx, val, typ, nullptr).first;
});
}


static void emit_typecheck(jl_codectx_t &ctx, const jl_cgval_t &x, jl_value_t *type, const std::string &msg)
{
Value *istype;
Expand Down Expand Up @@ -3005,42 +3030,16 @@ static Value *compute_box_tindex(jl_codectx_t &ctx, Value *datatype, jl_value_t
return tindex;
}

// Returns typeof(v), or null if v is a null pointer at run time.
// This is used when the value might have come from an undefined variable,
// yet we try to read its type to compute a union index when moving the value.
static Value *emit_typeof_or_null(jl_codectx_t &ctx, Value *v)
{
BasicBlock *nonnull = BasicBlock::Create(ctx.builder.getContext(), "nonnull", ctx.f);
BasicBlock *postBB = BasicBlock::Create(ctx.builder.getContext(), "postnull", ctx.f);
Value *isnull = ctx.builder.CreateICmpEQ(v, Constant::getNullValue(v->getType()));
ctx.builder.CreateCondBr(isnull, postBB, nonnull);
BasicBlock *entry = ctx.builder.GetInsertBlock();
ctx.builder.SetInsertPoint(nonnull);
Value *typof = emit_typeof(ctx, v);
ctx.builder.CreateBr(postBB);
nonnull = ctx.builder.GetInsertBlock(); // could have changed
ctx.builder.SetInsertPoint(postBB);
PHINode *ti = ctx.builder.CreatePHI(typof->getType(), 2);
ti->addIncoming(Constant::getNullValue(typof->getType()), entry);
ti->addIncoming(typof, nonnull);
return ti;
}

// get the runtime tindex value, assuming val is already converted to type typ if it has a TIndex
static Value *compute_tindex_unboxed(jl_codectx_t &ctx, const jl_cgval_t &val, jl_value_t *typ)
static Value *compute_tindex_unboxed(jl_codectx_t &ctx, const jl_cgval_t &val, jl_value_t *typ, bool maybenull=false)
{
if (val.typ == jl_bottom_type)
return UndefValue::get(getInt8Ty(ctx.builder.getContext()));
if (val.constant)
return ConstantInt::get(getInt8Ty(ctx.builder.getContext()), get_box_tindex((jl_datatype_t*)jl_typeof(val.constant), typ));

if (val.TIndex)
return ctx.builder.CreateAnd(val.TIndex, ConstantInt::get(getInt8Ty(ctx.builder.getContext()), 0x7f));
Value *typof;
if (val.isboxed && !jl_is_concrete_type(val.typ) && !jl_is_type_type(val.typ))
typof = emit_typeof_or_null(ctx, val.V);
else
typof = emit_typeof_boxed(ctx, val);
Value *typof = emit_typeof_boxed(ctx, val, maybenull);
return compute_box_tindex(ctx, typof, val.typ, typ);
}

Expand Down Expand Up @@ -3222,14 +3221,17 @@ static void emit_unionmove(jl_codectx_t &ctx, Value *dest, MDNode *tbaa_dst, con
Value *src_ptr = data_pointer(ctx, src);
unsigned nb = jl_datatype_size(typ);
unsigned alignment = julia_alignment(typ);
Value *nbytes = ConstantInt::get(getSizeTy(ctx.builder.getContext()), nb);
if (skip) {
// TODO: this Select is very bad for performance, but is necessary to work around LLVM bugs with the undef option that we want to use:
// select copy dest -> dest to simulate an undef value / conditional copy
// src_ptr = ctx.builder.CreateSelect(skip, dest, src_ptr);
nbytes = ctx.builder.CreateSelect(skip, Constant::getNullValue(getSizeTy(ctx.builder.getContext())), nbytes);
}
emit_memcpy(ctx, dest, tbaa_dst, src_ptr, src.tbaa, nbytes, alignment, isVolatile);
// TODO: this branch may be bad for performance, but is necessary to work around LLVM bugs with the undef option that we want to use:
// select copy dest -> dest to simulate an undef value / conditional copy
// if (skip) src_ptr = ctx.builder.CreateSelect(skip, dest, src_ptr);
auto f = [&] {
(void)emit_memcpy(ctx, dest, tbaa_dst, src_ptr, src.tbaa, nb, alignment, isVolatile);
return nullptr;
};
if (skip)
emit_guarded_test(ctx, skip, nullptr, f);
else
f();
}
}
}
Expand Down Expand Up @@ -3282,12 +3284,16 @@ static void emit_unionmove(jl_codectx_t &ctx, Value *dest, MDNode *tbaa_dst, con
}
else {
assert(src.isboxed && "expected boxed value for sizeof/alignment computation");
Value *datatype = emit_typeof_boxed(ctx, src);
Value *copy_bytes = emit_datatype_size(ctx, datatype);
if (skip) {
copy_bytes = ctx.builder.CreateSelect(skip, ConstantInt::get(copy_bytes->getType(), 0), copy_bytes);
}
emit_memcpy(ctx, dest, tbaa_dst, src, copy_bytes, /*TODO: min-align*/1, isVolatile);
auto f = [&] {
Value *datatype = emit_typeof_boxed(ctx, src);
Value *copy_bytes = emit_datatype_size(ctx, datatype);
emit_memcpy(ctx, dest, tbaa_dst, src, copy_bytes, /*TODO: min-align*/1, isVolatile);
return nullptr;
};
if (skip)
emit_guarded_test(ctx, skip, nullptr, f);
else
f();
}
}

Expand Down
30 changes: 18 additions & 12 deletions src/codegen.cpp
View file
Original file line number Diff line number Diff line change
Expand Up @@ -1816,7 +1816,7 @@ static jl_cgval_t convert_julia_type_union(jl_codectx_t &ctx, const jl_cgval_t &
if (!union_isaBB) {
union_isaBB = BasicBlock::Create(ctx.builder.getContext(), "union_isa", ctx.f);
ctx.builder.SetInsertPoint(union_isaBB);
union_box_dt = emit_typeof_or_null(ctx, v.Vboxed);
union_box_dt = emit_typeof(ctx, v.Vboxed, skip != NULL);
post_union_isaBB = ctx.builder.GetInsertBlock();
}
};
Expand Down Expand Up @@ -1915,22 +1915,27 @@ static jl_cgval_t convert_julia_type(jl_codectx_t &ctx, const jl_cgval_t &v, jl_
return ghostValue(ctx, typ);
Value *new_tindex = NULL;
if (jl_is_concrete_type(typ)) {
assert(skip == nullptr && "skip only valid for union type return");
if (v.TIndex && !jl_is_pointerfree(typ)) {
// discovered that this union-split type must actually be isboxed
if (v.Vboxed) {
return jl_cgval_t(v.Vboxed, nullptr, true, typ, NULL, ctx.tbaa());
}
else {
// type mismatch: there weren't any boxed values in the union
CreateTrap(ctx.builder);
if (skip)
*skip = ConstantInt::get(getInt1Ty(ctx.builder.getContext()), 1);
else
CreateTrap(ctx.builder);
return jl_cgval_t(ctx.builder.getContext());
}
}
if (jl_is_concrete_type(v.typ) && !jl_is_kind(v.typ)) {
if (jl_is_concrete_type(typ) && !jl_is_kind(typ)) {
// type mismatch: changing from one leaftype to another
CreateTrap(ctx.builder);
if (skip)
*skip = ConstantInt::get(getInt1Ty(ctx.builder.getContext()), 1);
else
CreateTrap(ctx.builder);
return jl_cgval_t(ctx.builder.getContext());
}
}
Expand Down Expand Up @@ -2938,7 +2943,7 @@ static bool emit_builtin_call(jl_codectx_t &ctx, jl_cgval_t *ret, jl_value_t *f,
}

else if (f == jl_builtin_typeof && nargs == 1) {
*ret = emit_typeof(ctx, argv[1]);
*ret = emit_typeof(ctx, argv[1], false);
return true;
}

Expand Down Expand Up @@ -4129,7 +4134,7 @@ static jl_cgval_t emit_sparam(jl_codectx_t &ctx, size_t i)
ctx.spvals_ptr,
i + sizeof(jl_svec_t) / sizeof(jl_value_t*));
Value *sp = tbaa_decorate(ctx.tbaa().tbaa_const, ctx.builder.CreateAlignedLoad(ctx.types().T_prjlvalue, bp, Align(sizeof(void*))));
Value *isnull = ctx.builder.CreateICmpNE(emit_typeof(ctx, sp),
Value *isnull = ctx.builder.CreateICmpNE(emit_typeof(ctx, sp, false),
track_pjlvalue(ctx, literal_pointer_val(ctx, (jl_value_t*)jl_tvar_type)));
jl_unionall_t *sparam = (jl_unionall_t*)ctx.linfo->def.method->sig;
for (size_t j = 0; j < i; j++) {
Expand Down Expand Up @@ -4204,7 +4209,7 @@ static jl_cgval_t emit_isdefined(jl_codectx_t &ctx, jl_value_t *sym)
ctx.spvals_ptr,
i + sizeof(jl_svec_t) / sizeof(jl_value_t*));
Value *sp = tbaa_decorate(ctx.tbaa().tbaa_const, ctx.builder.CreateAlignedLoad(ctx.types().T_prjlvalue, bp, Align(sizeof(void*))));
isnull = ctx.builder.CreateICmpNE(emit_typeof(ctx, sp),
isnull = ctx.builder.CreateICmpNE(emit_typeof(ctx, sp, false),
track_pjlvalue(ctx, literal_pointer_val(ctx, (jl_value_t*)jl_tvar_type)));
}
else {
Expand Down Expand Up @@ -4378,7 +4383,7 @@ static void emit_vi_assignment_unboxed(jl_codectx_t &ctx, jl_varinfo_t &vi, Valu
}
}
else {
emit_unionmove(ctx, vi.value.V, ctx.tbaa().tbaa_stack, rval_info, isboxed, vi.isVolatile);
emit_unionmove(ctx, vi.value.V, ctx.tbaa().tbaa_stack, rval_info, /*skip*/isboxed, vi.isVolatile);
}
}
}
Expand Down Expand Up @@ -4840,7 +4845,8 @@ static jl_cgval_t emit_expr(jl_codectx_t &ctx, jl_value_t *expr, ssize_t ssaval)
jl_error("GotoIfNot in value position");
}
if (jl_is_pinode(expr)) {
return convert_julia_type(ctx, emit_expr(ctx, jl_fieldref_noalloc(expr, 0)), jl_fieldref_noalloc(expr, 1));
Value *skip = NULL;
return convert_julia_type(ctx, emit_expr(ctx, jl_fieldref_noalloc(expr, 0)), jl_fieldref_noalloc(expr, 1), &skip);
}
if (!jl_is_expr(expr)) {
int needroot = true;
Expand Down Expand Up @@ -7717,7 +7723,7 @@ static jl_llvm_functions_t
else {
// must be careful to emit undef here (rather than a bitcast or
// load of val) if the runtime type of val isn't phiType
Value *isvalid = emit_isa(ctx, val, phiType, NULL).first;
Value *isvalid = emit_isa_and_defined(ctx, val, phiType);
V = emit_guarded_test(ctx, isvalid, undef_value_for_type(VN->getType()), [&] {
return emit_unbox(ctx, VN->getType(), val, phiType);
});
Expand All @@ -7729,7 +7735,7 @@ static jl_llvm_functions_t
// must be careful to emit undef here (rather than a bitcast or
// load of val) if the runtime type of val isn't phiType
assert(lty != ctx.types().T_prjlvalue);
Value *isvalid = emit_isa(ctx, val, phiType, NULL).first;
Value *isvalid = emit_isa_and_defined(ctx, val, phiType);
emit_guarded_test(ctx, isvalid, nullptr, [&] {
(void)emit_unbox(ctx, lty, val, phiType, maybe_decay_tracked(ctx, dest), ctx.tbaa().tbaa_stack);
return nullptr;
Expand Down Expand Up @@ -7771,7 +7777,7 @@ static jl_llvm_functions_t
RTindex = new_union.TIndex;
if (!RTindex) {
assert(new_union.isboxed && new_union.Vboxed && "convert_julia_type failed");
RTindex = compute_tindex_unboxed(ctx, new_union, phiType);
RTindex = compute_tindex_unboxed(ctx, new_union, phiType, true);
if (dest) {
// If dest is not set, this is a ghost union, the recipient of which
// is often not prepared to handle a boxed representation of the ghost.
Expand Down
3 changes: 2 additions & 1 deletion src/llvm-late-gc-lowering.cpp
View file
Original file line number Diff line number Diff line change
Expand Up @@ -1071,8 +1071,9 @@ void RecursivelyVisit(callback f, Value *V) {
if (isa<VisitInst>(TheUser))
f(VU);
if (isa<CallInst>(TheUser) || isa<LoadInst>(TheUser) ||
isa<SelectInst>(TheUser) || isa<PHINode>(TheUser) ||
isa<SelectInst>(TheUser) || isa<PHINode>(TheUser) || // TODO: should these be removed from this list?
isa<StoreInst>(TheUser) || isa<PtrToIntInst>(TheUser) ||
isa<ICmpInst>(TheUser) || // ICmpEQ/ICmpNE can be used with ptr types
isa<AtomicCmpXchgInst>(TheUser) || isa<AtomicRMWInst>(TheUser))
continue;
if (isa<GetElementPtrInst>(TheUser) || isa<BitCastInst>(TheUser) || isa<AddrSpaceCastInst>(TheUser)) {
Expand Down

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