* [pushed] c++: only cache constexpr calls that are constant exprs
@ 2023-07-17 21:18 Jason Merrill
2023-07-17 23:57 ` [FYI] c++: check for trying to cache non-constant expressions Jason Merrill
0 siblings, 1 reply; 2+ messages in thread
From: Jason Merrill @ 2023-07-17 21:18 UTC (permalink / raw)
To: gcc-patches
Tested x86_64-pc-linux-gnu, applying to trunk.
-- 8< --
In reviewing Nathaniel's patch for PR70331, it occurred to me that instead
of looking for various specific problematic things in the result of a
constexpr call to decide whether to cache it, we should use
reduced_constant_expression_p.
The change to that function is to avoid crashing on uninitialized objects of
non-class type.
In a trial version of this patch I checked to see what cases this stopped
caching; some were instances of partially-initialized return values, which
seem fine to not cache. Some were returning pointers to expiring local
variables, which we definitely want not to cache. And one was bit-cast3.C,
which will be handled in a follow-up patch.
gcc/cp/ChangeLog:
* constexpr.cc (cxx_eval_call_expression): Only cache
reduced_constant_expression_p results.
(reduced_constant_expression_p): Handle CONSTRUCTOR of scalar type.
(cxx_eval_constant_expression): Fold vectors here.
(cxx_eval_bare_aggregate): Not here.
---
gcc/cp/constexpr.cc | 17 +++++++++--------
1 file changed, 9 insertions(+), 8 deletions(-)
diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index c6f323ebf43..9d85c3be5cc 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -3033,7 +3033,7 @@ cxx_eval_call_expression (const constexpr_ctx *ctx, tree t,
}
else
{
- bool cacheable = true;
+ bool cacheable = !!entry;
if (result && result != error_mark_node)
/* OK */;
else if (!DECL_SAVED_TREE (fun))
@@ -3185,7 +3185,7 @@ cxx_eval_call_expression (const constexpr_ctx *ctx, tree t,
for the constexpr evaluation and should not be cached.
It is fine if the call allocates something and deallocates it
too. */
- if (entry
+ if (cacheable
&& (save_heap_alloc_count != ctx->global->heap_vars.length ()
|| (save_heap_dealloc_count
!= ctx->global->heap_dealloc_count)))
@@ -3204,10 +3204,6 @@ cxx_eval_call_expression (const constexpr_ctx *ctx, tree t,
cacheable = false;
break;
}
- /* Also don't cache a call that returns a deallocated pointer. */
- if (cacheable && (cp_walk_tree_without_duplicates
- (&result, find_heap_var_refs, NULL)))
- cacheable = false;
}
/* Rewrite all occurrences of the function's RESULT_DECL with the
@@ -3217,6 +3213,10 @@ cxx_eval_call_expression (const constexpr_ctx *ctx, tree t,
&& !is_empty_class (TREE_TYPE (res)))
if (replace_decl (&result, res, ctx->object))
cacheable = false;
+
+ /* Only cache a permitted result of a constant expression. */
+ if (cacheable && !reduced_constant_expression_p (result))
+ cacheable = false;
}
else
/* Couldn't get a function copy to evaluate. */
@@ -3268,8 +3268,9 @@ reduced_constant_expression_p (tree t)
case CONSTRUCTOR:
/* And we need to handle PTRMEM_CST wrapped in a CONSTRUCTOR. */
tree field;
- if (TREE_CODE (TREE_TYPE (t)) == VECTOR_TYPE)
- /* An initialized vector would have a VECTOR_CST. */
+ if (!AGGREGATE_TYPE_P (TREE_TYPE (t)))
+ /* A constant vector would be folded to VECTOR_CST.
+ A CONSTRUCTOR of scalar type means uninitialized. */
return false;
if (CONSTRUCTOR_NO_CLEARING (t))
{
base-commit: caabf0973a4e9a26421c94d540e3e20051e93e77
--
2.39.3
^ permalink raw reply [flat|nested] 2+ messages in thread
* [FYI] c++: check for trying to cache non-constant expressions
2023-07-17 21:18 [pushed] c++: only cache constexpr calls that are constant exprs Jason Merrill
@ 2023-07-17 23:57 ` Jason Merrill
0 siblings, 0 replies; 2+ messages in thread
From: Jason Merrill @ 2023-07-17 23:57 UTC (permalink / raw)
To: gcc-patches
Just for reference, not applying.
-- 8< --
This is what I used to check for what non-constant results we were
previously caching. After the previous two patches it's just address
of a local and a partially-uninitialized COMPLEX_EXPR.
gcc/cp/ChangeLog:
* constexpr.cc (cxx_eval_call_expression): Add checking.
---
gcc/cp/constexpr.cc | 21 +++++++++++++++++----
1 file changed, 17 insertions(+), 4 deletions(-)
diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index 6e8f1c2b61e..ed4e6a3acf9 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -3206,6 +3206,11 @@ cxx_eval_call_expression (const constexpr_ctx *ctx, tree t,
cacheable = false;
break;
}
+ /* Also don't cache a call that returns a deallocated pointer. */
+ if (cacheable && CHECKING_P
+ && (cp_walk_tree_without_duplicates
+ (&result, find_heap_var_refs, NULL)))
+ cacheable = false;
}
/* Rewrite all occurrences of the function's RESULT_DECL with the
@@ -3215,10 +3220,6 @@ cxx_eval_call_expression (const constexpr_ctx *ctx, tree t,
&& !is_empty_class (TREE_TYPE (res)))
if (replace_decl (&result, res, ctx->object))
cacheable = false;
-
- /* Only cache a permitted result of a constant expression. */
- if (cacheable && !reduced_constant_expression_p (result))
- cacheable = false;
}
else
/* Couldn't get a function copy to evaluate. */
@@ -3230,6 +3231,18 @@ cxx_eval_call_expression (const constexpr_ctx *ctx, tree t,
result = error_mark_node;
else if (!result)
result = void_node;
+
+ /* Only cache a permitted result of a constant expression. */
+ if (cacheable && !reduced_constant_expression_p (result))
+ {
+ cacheable = false;
+ if (CHECKING_P
+ && result != void_node && result != error_mark_node
+ && !(TREE_CODE (result) == CONSTRUCTOR
+ && CONSTRUCTOR_NO_CLEARING (result)))
+ internal_error ("caching %qE", result);
+ }
+
if (entry)
entry->result = cacheable ? result : error_mark_node;
}
base-commit: c7ac1de5f5c561b2d90c084a638c232d322d54e6
--
2.39.3
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