[PATCH, i386]: Emit cld instruction when stringops are used

["Uros Bizjak" <ubizjak at gmail dot com>]

Hello!

This patch adds back generation of cld instructions when stringops are
used in the function. cld insn is emitted as unspec volatile at the
end of function prologue. Currently, patched gcc emits cld when
"-mcld" is added to compile flags.

Currently, there is no configure-time support, I think we should first
reach a consensus on how to configure this. We can

./configure --with-cld (or something), where no cld generation is default.

Anyway the code for:

--cut here--
extern void *memcpy (void *, const void *, __SIZE_TYPE__);

int foo[10];
int bar[10];

char baz[15];
char quux[15];

void
do_copy ()
{
  memcpy (foo, bar, sizeof foo);
  memcpy (baz, quux, sizeof baz);
}
--cut here--

now generates (gcc -Os):

do_copy:
        pushl   %ebp
        movl    %esp, %ebp
        pushl   %edi
        pushl   %esi
        subl    $8, %esp
        cld                                       <- here
        movl    $foo, %edi
        movl    $bar, %esi
        movl    $10, %ecx
        rep movsl
        movl    $baz, %edi
        movl    $quux, %esi
        movb    $15, %cl
        rep movsb
        popl    %eax
        popl    %edx
        popl    %esi
        popl    %edi
        popl    %ebp
        ret

2008-03-06  Uros Bizjak  <ubizjak@gmail.com>

	* config/i386/i386.h (TARGET_CLD): New define.
	(struct machine_function): Add needs_cld field.
	(ix86_current_function_needs_cld): New define.
	* config/i386/i386.md (UNSPEC_CLD): New unspec volatile constant.
	("cld"): New isns pattern.
	("strmov_singleop"): Set ix86_current_function_needs_cld flag.
	("rep_mov"): Ditto.
	("strset_singleop"): Ditto.
	("rep_stos"): Ditto.
	("cmpstrnqi_nz_1"): Ditto.
	("cmpstrnqi_1"): Ditto.
	("strlenqi_1"): Ditto.
	* config/i386/i386.opt (mcld): New option.
	* config/i386/i386.c (ix86_expand_prologue): Emit cld insn for
	TARGET_CLD when ix86_current_function_needs_cld is set.

The patch was bootstrapped on i686-pc-linux-gnu, regression test with
-mcld is in progress.

Uros.

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