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gh-114058: Only allow immortal constants in tier 2 #115817

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gh-114058: Only allow immortal constants in tier 2 #115817

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05a604f
Only allow immortal constants in tier 2
Fidget-Spinner Feb 22, 2024
38ee21c
Allow borrowed ref for _LOAD_CONST_INLINE
Fidget-Spinner Feb 22, 2024
c75030d
update comments
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3ff15c7
make flags simpler
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c447000
Merge remote-tracking branch 'upstream/main' into fix_redundancy_refl…
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44 changes: 32 additions & 12 deletions Python/optimizer_analysis.c
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Original file line number Diff line number Diff line change
Expand Up @@ -164,10 +164,6 @@ abstractcontext_fini(_Py_UOpsAbstractInterpContext *ctx)
return;
}
ctx->curr_frame_depth = 0;
int tys = ctx->t_arena.ty_curr_number;
for (int i = 0; i < tys; i++) {
Py_CLEAR(ctx->t_arena.arena[i].const_val);
}
}

static int
Expand Down Expand Up @@ -218,7 +214,8 @@ ctx_frame_pop(
}


// Takes a borrowed reference to const_val, turns that into a strong reference.
// Only accepts immortal constants or borrowed constants that will be kept alive
// elsewhere.
static _Py_UOpsSymType*
sym_new(_Py_UOpsAbstractInterpContext *ctx,
PyObject *const_val)
Expand All @@ -230,13 +227,9 @@ sym_new(_Py_UOpsAbstractInterpContext *ctx,
return NULL;
}
ctx->t_arena.ty_curr_number++;
self->const_val = NULL;
self->typ = NULL;
self->flags = 0;

if (const_val != NULL) {
self->const_val = Py_NewRef(const_val);
}
self->const_val = const_val;

return self;
}
Expand Down Expand Up @@ -317,7 +310,7 @@ sym_new_known_type(_Py_UOpsAbstractInterpContext *ctx,
return res;
}

// Takes a borrowed reference to const_val.
// Decrefs const_val if it is not immortal and discards it.
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This seems like an interface that is hard to use. Let the caller worry about mortality.
This also contradicts the comment on sym_new that it is OK to pass in mortal objects as long as the caller retains a strong reference.

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That way is more complex to handle error conditions.
E.g
For newly created objects, out of space means we should decref it, but the code isn't as clean as

OUT_OF_SPACE_IF_NULL(res = sym_new_const(ctx, temp));

but rather becomes

if (res = sym_new_const(ctx, temp) == NULL) {
    Py_DECREF(temp);
    goto out_of_space;
}

Then we need to repeat that on every call site.

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Either temp is immortal, in which case the caller need do nothing, or it is mortal in which case it needs proper handling.

Correctness is paramount here, as bugs are going to be very hard to reproduce. Reproducing bugs in the T1 specializer is hard enough. We need to extra careful here. A bit of verbosity is a small price to pay for being able to reason about the code.

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The proposed code here would totally invalidate _BINARY_OP_ADD_FLOAT and other float-producing uops (as well as int-producing uops if the value is not a cached short int -- which is a distinction easily overlooked when testing).

static inline _Py_UOpsSymType*
sym_new_const(_Py_UOpsAbstractInterpContext *ctx, PyObject *const_val)
{
Expand All @@ -330,9 +323,36 @@ sym_new_const(_Py_UOpsAbstractInterpContext *ctx, PyObject *const_val)
return NULL;
}
sym_set_type(temp, Py_TYPE(const_val));
sym_set_flag(temp, TRUE_CONST);
sym_set_flag(temp, KNOWN);
sym_set_flag(temp, NOT_NULL);
// If the constant is not immortal, discard it.
if (_Py_IsImmortal(const_val)) {
sym_set_flag(temp, TRUE_CONST);
}
else {
Py_DECREF(const_val);
temp->const_val = NULL;
}
return temp;
}

// Same as sym_new_const, but borrows const_val, assuming it will be kept alive.
// Only safe if we know const_val has a strong reference elsewhere.
static inline _Py_UOpsSymType*
sym_new_const_borrow(_Py_UOpsAbstractInterpContext *ctx, PyObject *const_val)
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I think you should scrap this, and leave it to the caller to handle lifetimes.

{
assert(const_val != NULL);
_Py_UOpsSymType *temp = sym_new(
ctx,
const_val
);
if (temp == NULL) {
return NULL;
}
sym_set_type(temp, Py_TYPE(const_val));
sym_set_flag(temp, KNOWN);
sym_set_flag(temp, NOT_NULL);
sym_set_flag(temp, TRUE_CONST);
return temp;
}

Expand Down
4 changes: 2 additions & 2 deletions Python/tier2_redundancy_eliminator_bytecodes.c
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Original file line number Diff line number Diff line change
Expand Up @@ -204,15 +204,15 @@ dummy_func(void) {
}

op(_LOAD_CONST_INLINE, (ptr/4 -- value)) {
OUT_OF_SPACE_IF_NULL(value = sym_new_const(ctx, ptr));
OUT_OF_SPACE_IF_NULL(value = sym_new_const_borrow(ctx, ptr));
}

op(_LOAD_CONST_INLINE_BORROW, (ptr/4 -- value)) {
OUT_OF_SPACE_IF_NULL(value = sym_new_const(ctx, ptr));
}

op(_LOAD_CONST_INLINE_WITH_NULL, (ptr/4 -- value, null)) {
OUT_OF_SPACE_IF_NULL(value = sym_new_const(ctx, ptr));
OUT_OF_SPACE_IF_NULL(value = sym_new_const_borrow(ctx, ptr));
OUT_OF_SPACE_IF_NULL(null = sym_new_null(ctx));
}

Expand Down
4 changes: 2 additions & 2 deletions Python/tier2_redundancy_eliminator_cases.c.h
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