On Sat, Sep 14, 2024 at 5:31 AM Siddhesh Poyarekar <siddh...@gotplt.org> wrote:
>
> In PTR + OFFSET cases, try harder to see if the target offset could
> result in a constant. Specifically, if the offset is a PHI node with
> all constant branches, return the minimum (or maximum for OST_MINIMUM)
> of the possible values.
>
> gcc/ChangeLog:
>
> PR tree-optimization/116556
> * tree-object-size.cc (try_collapsing_offset): New function.
> (plus_stmt_object_size): Use it.
> * gcc/testsuite/gcc.dg/builtin-object-size-1.c (test12): New
> function.
> (main): Call it.
>
> Signed-off-by: Siddhesh Poyarekar <siddh...@gotplt.org>
> ---
> Tests underway for x86_64 and i686. OK if they pass?
>
> gcc/testsuite/gcc.dg/builtin-object-size-1.c | 25 ++++++++++++
> gcc/tree-object-size.cc | 41 ++++++++++++++++++++
> 2 files changed, 66 insertions(+)
>
> diff --git a/gcc/testsuite/gcc.dg/builtin-object-size-1.c
> b/gcc/testsuite/gcc.dg/builtin-object-size-1.c
> index d6d13c5ef7a..eed1653505c 100644
> --- a/gcc/testsuite/gcc.dg/builtin-object-size-1.c
> +++ b/gcc/testsuite/gcc.dg/builtin-object-size-1.c
> @@ -712,6 +712,30 @@ test11 (void)
> }
> #endif
>
> +void
> +__attribute__ ((noinline))
> +test12 (unsigned cond)
> +{
> + char *buf2 = malloc (10);
> + char *p;
> + size_t t;
> +
> + if (cond)
> + t = 8;
> + else
> + t = 4;
> +
> + p = &buf2[t];
> +
> +#ifdef __builtin_object_size
> + if (__builtin_object_size (p, 0) != (cond ? 2 : 6))
> + FAIL ();
> +#else
> + if (__builtin_object_size (p, 0) != 6)
> + FAIL ();
> +#endif
> +}
> +
> int
> main (void)
> {
> @@ -729,6 +753,7 @@ main (void)
> test10 ();
> #ifndef SKIP_STRNDUP
> test11 ();
> + test12 (1);
> #endif
> DONE ();
> }
> diff --git a/gcc/tree-object-size.cc b/gcc/tree-object-size.cc
> index 4c1fa9b555f..d90cc43fa97 100644
> --- a/gcc/tree-object-size.cc
> +++ b/gcc/tree-object-size.cc
> @@ -1467,6 +1467,44 @@ merge_object_sizes (struct object_size_info *osi, tree
> dest, tree orig)
> return bitmap_bit_p (osi->reexamine, SSA_NAME_VERSION (orig));
> }
>
> +/* For constant sizes, try collapsing a non-constant offset to a constant if
> + possible. The case handled at the moment is when the offset is a PHI node
> + with all of its targets are constants. */
> +
> +static tree
> +try_collapsing_offset (tree op, int object_size_type)
> +{
> + gcc_assert (!(object_size_type & OST_DYNAMIC));
> +
> + if (TREE_CODE (op) != SSA_NAME)
> + return op;
> +
> + gimple *stmt = SSA_NAME_DEF_STMT (op);
> +
> + if (gimple_code (stmt) != GIMPLE_PHI)
> + return op;
> +
> + tree off = size_unknown (object_size_type);
> +
> + for (unsigned i = 0; i < gimple_phi_num_args (stmt); i++)
> + {
> + tree rhs = gimple_phi_arg (stmt, i)->def;
> +
> + if (TREE_CODE (rhs) != INTEGER_CST)
> + return op;
> +
> + /* Note that this is the *opposite* of what we usually do with sizes,
> + because the maximum offset estimate here will give us a minimum size
> + estimate and vice versa. */
> + enum tree_code code = (object_size_type & OST_MINIMUM
> + ? MAX_EXPR : MIN_EXPR);
> +
> + off = size_binop (code, off, rhs);
I suspect this won't work for integer constants which have the what
would be the sign bit set.
That is:
```
void
__attribute__ ((noinline))
test9 (unsigned cond)
{
char *buf2 = __builtin_malloc (10);
char *p;
__SIZE_TYPE__ t;
if (cond)
t = -4;
else
t = 4;
p = &buf2[4] + t;
if (__builtin_object_size (&p[0], 0) != 10)
__builtin_abort ();
}
```
Since you do the MIN/MAX in unsigned.
Thanks,
Andrew Pinski
> + }
> +
> + gcc_assert (TREE_CODE (off) == INTEGER_CST);
> + return off;
> +}
>
> /* Compute object_sizes for VAR, defined to the result of an assignment
> with operator POINTER_PLUS_EXPR. Return true if the object size might
> @@ -1499,6 +1537,9 @@ plus_stmt_object_size (struct object_size_info *osi,
> tree var, gimple *stmt)
> if (object_sizes_unknown_p (object_size_type, varno))
> return false;
>
> + if (!(object_size_type & OST_DYNAMIC) && TREE_CODE (op1) != INTEGER_CST)
> + op1 = try_collapsing_offset (op1, object_size_type);
> +
> /* Handle PTR + OFFSET here. */
> if (size_valid_p (op1, object_size_type)
> && (TREE_CODE (op0) == SSA_NAME || TREE_CODE (op0) == ADDR_EXPR))
> --
> 2.45.1
>
