On 25 November 2016 at 21:17, Jeff Law <l...@redhat.com> wrote:
> On 11/25/2016 01:07 AM, Richard Biener wrote:
>
>>> For the tail-call, issue should we artificially create a lhs and use that
>>> as return value (perhaps by a separate pass before tailcall) ?
>>>
>>> __builtin_memcpy (a1, a2, a3);
>>> return a1;
>>>
>>> gets transformed to:
>>> _1 = __builtin_memcpy (a1, a2, a3)
>>> return _1;
>>>
>>> So tail-call optimization pass would see the IL in it's expected form.
>>
>>
>> As said, a RTL expert needs to chime in here. Iff then tail-call
>> itself should do this rewrite. But if this form is required to make
>> things work (I suppose you checked it _does_ actually work?) then
>> we'd need to make sure later passes do not undo it. So it looks
>> fragile to me. OTOH I seem to remember that the flags we set on
>> GIMPLE are merely a hint to RTL expansion and the tailcalling is
>> verified again there?
>
> So tail calling actually sits on the border between trees and RTL.
> Essentially it's an expand-time decision as we use information from trees as
> well as low level target information.
>
> I would not expect the former sequence to tail call. The tail calling code
> does not know that the return value from memcpy will be a1. Thus the tail
> calling code has to assume that it'll have to copy a1 into the return
> register after returning from memcpy, which obviously can't be done if we
> tail called memcpy.
>
> The second form is much more likely to turn into a tail call sequence
> because the return value from memcpy will be sitting in the proper register.
> This form out to work for most calling conventions that allow tail calls.
>
> We could (in theory) try and exploit the fact that memcpy returns its first
> argument as a return value, but that would only be helpful on a target where
> the first argument and return value use the same register. So I'd have a
> slight preference to rewriting per Prathamesh's suggestion above since it's
> more general.
Thanks for the suggestion. The attached patch creates artificial lhs,
and returns it if the function returns it's argument and that argument
is used as return-value.
eg:
f (void * a1, void * a2, long unsigned int a3)
{
<bb 2> [0.0%]:
# .MEM_5 = VDEF <.MEM_1(D)>
__builtin_memcpy (a1_2(D), a2_3(D), a3_4(D));
# VUSE <.MEM_5>
return a1_2(D);
}
is transformed to:
f (void * a1, void * a2, long unsigned int a3)
{
void * _6;
<bb 2> [0.0%]:
# .MEM_5 = VDEF <.MEM_1(D)>
_6 = __builtin_memcpy (a1_2(D), a2_3(D), a3_4(D));
# VUSE <.MEM_5>
return _6;
}
While testing, I came across an issue with function f() defined
intail-padding1.C:
struct X
{
~X() {}
int n;
char d;
};
X f()
{
X nrvo;
__builtin_memset (&nrvo, 0, sizeof(X));
return nrvo;
}
input to the pass:
X f() ()
{
<bb 2> [0.0%]:
# .MEM_3 = VDEF <.MEM_1(D)>
__builtin_memset (nrvo_2(D), 0, 8);
# VUSE <.MEM_3>
return nrvo_2(D);
}
verify_gimple_return failed with:
tail-padding1.C:13:1: error: invalid conversion in return statement
}
^
struct X
struct X &
# VUSE <.MEM_3>
return _4;
It seems the return type of function (struct X) differs with the type
of return value (struct X&).
Not sure how this is possible ?
To work around that, I guarded the transform on:
useless_type_conversion_p (TREE_TYPE (TREE_TYPE (cfun->decl)),
TREE_TYPE (retval)))
in the patch. Does that look OK ?
Bootstrap+tested on x86_64-unknown-linux-gnu with --enable-languages=all,ada.
Cross-tested on arm*-*-*, aarch64*-*-*.
Thanks,
Prathamesh
>
>
> Jeff
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/tailcall-9.c
b/gcc/testsuite/gcc.dg/tree-ssa/tailcall-9.c
new file mode 100644
index 0000000..b3fdc6c
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/tailcall-9.c
@@ -0,0 +1,10 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-tailc-details" } */
+
+void *f(void *a1, void *a2, __SIZE_TYPE__ a3)
+{
+ __builtin_memcpy (a1, a2, a3);
+ return a1;
+}
+
+/* { dg-final { scan-tree-dump-times "Found tail call" 1 "tailc" } } */
diff --git a/gcc/tree-tailcall.c b/gcc/tree-tailcall.c
index 66a0a4c..d46ca50 100644
--- a/gcc/tree-tailcall.c
+++ b/gcc/tree-tailcall.c
@@ -401,6 +401,7 @@ find_tail_calls (basic_block bb, struct tailcall **ret)
basic_block abb;
size_t idx;
tree var;
+ greturn *ret_stmt = NULL;
if (!single_succ_p (bb))
return;
@@ -408,6 +409,8 @@ find_tail_calls (basic_block bb, struct tailcall **ret)
for (gsi = gsi_last_bb (bb); !gsi_end_p (gsi); gsi_prev (&gsi))
{
stmt = gsi_stmt (gsi);
+ if (!ret_stmt)
+ ret_stmt = dyn_cast<greturn *> (stmt);
/* Ignore labels, returns, nops, clobbers and debug stmts. */
if (gimple_code (stmt) == GIMPLE_LABEL
@@ -422,6 +425,37 @@ find_tail_calls (basic_block bb, struct tailcall **ret)
{
call = as_a <gcall *> (stmt);
ass_var = gimple_call_lhs (call);
+ if (!ass_var)
+ {
+ /* Check if function returns one if it's arguments
+ and that argument is used as return value.
+ In that case create an artificial lhs to call_stmt,
+ and set it as the return value. */
+
+ unsigned rf = gimple_call_return_flags (call);
+ if (rf & ERF_RETURNS_ARG)
+ {
+ unsigned argnum = rf & ERF_RETURN_ARG_MASK;
+ if (argnum < gimple_call_num_args (call)
+ && ret_stmt)
+ {
+ tree arg = gimple_call_arg (call, argnum);
+ tree retval = gimple_return_retval (ret_stmt);
+ if (retval
+ && TREE_CODE (retval) == SSA_NAME
+ && operand_equal_p (retval, arg, 0)
+ && useless_type_conversion_p (TREE_TYPE (TREE_TYPE
(cfun->decl)),
+ TREE_TYPE (retval)))
+ {
+ ass_var = make_ssa_name (TREE_TYPE (arg));
+ gimple_call_set_lhs (call, ass_var);
+ update_stmt (call);
+ gimple_return_set_retval (ret_stmt, ass_var);
+ update_stmt (ret_stmt);
+ }
+ }
+ }
+ }
break;
}
