On Tue, Feb 28, 2017 at 03:33:11PM +0530, Prathamesh Kulkarni wrote:
> Hi,
> The attached patch adds special-casing for strcpy/strncpy to dse pass.
> Bootstrapped+tested on x86_64-unknown-linux-gnu.
> OK for GCC 8 ?
What is special on strcpy/strncpy? Unlike memcpy/memmove/memset, you don't
know the length they store (at least not in general), you don't know the
value, all you know is that they are for the first argument plain store
without remembering the pointer or anything based on it anywhere except in
the return value.
I believe stpcpy, stpncpy, strcat, strncat at least have the same behavior.
On the other side, without knowing the length of the store, you can't treat
it as killing something (ok, some calls like strcpy or stpcpy store at least
the first byte).
> diff --git a/gcc/testsuite/gcc.dg/tree-ssa/pr79715.c
> b/gcc/testsuite/gcc.dg/tree-ssa/pr79715.c
> new file mode 100644
> index 0000000..1a832ca
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/tree-ssa/pr79715.c
> @@ -0,0 +1,12 @@
> +/* { dg-do compile } */
> +/* { dg-options "-O2 -fdump-tree-dse-details" } */
> +
> +void f(const char *s)
> +{
> + unsigned n = __builtin_strlen (s) + 1;
> + char *p = __builtin_malloc (n);
> + __builtin_strcpy (p, s);
> + __builtin_free (p);
> +}
> +
> +/* { dg-final { scan-tree-dump "Deleted dead call: __builtin_strcpy" "dse1"
> } } */
> diff --git a/gcc/tree-ssa-dse.c b/gcc/tree-ssa-dse.c
> index 53feaf3..6d3c3c3 100644
> --- a/gcc/tree-ssa-dse.c
> +++ b/gcc/tree-ssa-dse.c
> @@ -97,6 +97,8 @@ initialize_ao_ref_for_dse (gimple *stmt, ao_ref *write)
> case BUILT_IN_MEMCPY:
> case BUILT_IN_MEMMOVE:
> case BUILT_IN_MEMSET:
> + case BUILT_IN_STRNCPY:
> + case BUILT_IN_STRCPY:
> {
> tree size = NULL_TREE;
> if (gimple_call_num_args (stmt) == 3)
> @@ -395,6 +397,8 @@ maybe_trim_memstar_call (ao_ref *ref, sbitmap live,
> gimple *stmt)
> {
> case BUILT_IN_MEMCPY:
> case BUILT_IN_MEMMOVE:
> + case BUILT_IN_STRNCPY:
> + case BUILT_IN_STRCPY:
> {
> int head_trim, tail_trim;
> compute_trims (ref, live, &head_trim, &tail_trim, stmt);
> @@ -713,6 +717,7 @@ dse_dom_walker::dse_optimize_stmt (gimple_stmt_iterator
> *gsi)
> case BUILT_IN_MEMCPY:
> case BUILT_IN_MEMMOVE:
> case BUILT_IN_MEMSET:
> + case BUILT_IN_STRNCPY:
> {
> /* Occasionally calls with an explicit length of zero
> show up in the IL. It's pointless to do analysis
> @@ -723,7 +728,10 @@ dse_dom_walker::dse_optimize_stmt (gimple_stmt_iterator
> *gsi)
> delete_dead_call (gsi);
> return;
> }
> -
> + }
> + /* fallthru */
> + case BUILT_IN_STRCPY:
> + {
> gimple *use_stmt;
> enum dse_store_status store_status;
> m_byte_tracking_enabled
Jakub
