[PATCH try 2 resend] [i386] Remove warnings for ignoring -mcall-ms2sysv-xlogues.

[Daniel Santos <daniel dot santos at pobox dot com>]

I appear to have forgotten to cc gcc-patches, sorry about that.

There are currently three cases where we issue a warning when disabling
-mcall-ms2sysv-xlogues for a function, but I never added a proper
warning, so there's no mechanism for disabling it.  This is something
that I meant to address sooner.  I'm thinking that it's better to just
remove the warning entirely and document these cases, rather than adding
a new warning.  Any thoughts?

These are the conditions:

* the use of -fsplit-stack,
* the use of static call chains (not sure if we can ever have that), and
* if the function calls __buildin_eh_return.

Some of these cases can likely be supported, but they are just on the
"not yet tested" list.

2017-06-11  Daniel Santos  <daniel.santos@pobox.com

	* config/i386/i386.c (warn_once_call_ms2sysv_xlogues): Remove.
	(ix86_compute_frame_layout): Don't call warn_once_call_ms2sysv_xlogues.
	(ix86_expand_call): Likewise.

Thanks,
Daniel

Signed-off-by: Daniel Santos <daniel.santos@pobox.com>
---
 gcc/config/i386/i386.c | 26 +++-----------------------
 gcc/doc/invoke.texi    | 25 ++++++++++++++++++++-----
 2 files changed, 23 insertions(+), 28 deletions(-)

diff --git a/gcc/config/i386/i386.c b/gcc/config/i386/i386.c
index d5c2d46bf5e..2dc6e53c765 100644
--- a/gcc/config/i386/i386.c
+++ b/gcc/config/i386/i386.c
@@ -12772,18 +12772,6 @@ ix86_builtin_setjmp_frame_value (void)
   return stack_realign_fp ? hard_frame_pointer_rtx : virtual_stack_vars_rtx;
 }
 
-/* Emits a warning for unsupported msabi to sysv pro/epilogues.  */
-static void warn_once_call_ms2sysv_xlogues (const char *feature)
-{
-  static bool warned_once = false;
-  if (!warned_once)
-    {
-      warning (0, "-mcall-ms2sysv-xlogues is not compatible with %s",
-	       feature);
-      warned_once = true;
-    }
-}
-
 /* When using -fsplit-stack, the allocation routines set a field in
    the TCB to the bottom of the stack plus this much space, measured
    in bytes.  */
@@ -12814,18 +12802,10 @@ ix86_compute_frame_layout (void)
       gcc_assert (TARGET_SSE);
       gcc_assert (!ix86_using_red_zone ());
 
-      if (crtl->calls_eh_return)
+      if (crtl->calls_eh_return || ix86_static_chain_on_stack)
 	{
 	  gcc_assert (!reload_completed);
 	  m->call_ms2sysv = false;
-	  warn_once_call_ms2sysv_xlogues ("__builtin_eh_return");
-	}
-
-      else if (ix86_static_chain_on_stack)
-	{
-	  gcc_assert (!reload_completed);
-	  m->call_ms2sysv = false;
-	  warn_once_call_ms2sysv_xlogues ("static call chains");
 	}
 
       /* Finally, compute which registers the stub will manage.  */
@@ -29290,9 +29270,9 @@ ix86_expand_call (rtx retval, rtx fnaddr, rtx callarg1,
 	  else if (ix86_function_ms_hook_prologue (current_function_decl))
 	    ;
 
-	  /* TODO: Cases not yet examined.  */
+	  /* TODO: Compatibility not yet examined.  */
 	  else if (flag_split_stack)
-	    warn_once_call_ms2sysv_xlogues ("-fsplit-stack");
+	    ;
 
 	  else
 	    {
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index c1168823af7..eec02b43a4f 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -25389,11 +25389,26 @@ using the function attributes @code{ms_abi} and @code{sysv_abi}.
 @opindex mno-call-ms2sysv-xlogues
 Due to differences in 64-bit ABIs, any Microsoft ABI function that calls a
 System V ABI function must consider RSI, RDI and XMM6-15 as clobbered.  By
-default, the code for saving and restoring these registers is emitted inline,
-resulting in fairly lengthy prologues and epilogues.  Using
-@option{-mcall-ms2sysv-xlogues} emits prologues and epilogues that
-use stubs in the static portion of libgcc to perform these saves and restores,
-thus reducing function size at the cost of a few extra instructions.
+default, the instructions for saving and restoring these registers are emitted
+inline, resulting in fairly lengthy pro- and epilogues.  Using
+@option{-mcall-ms2sysv-xlogues} emits pro- and epilogues that use stubs in the
+static portion of libgcc to perform these saves and restores, thus reducing
+function size at the cost of executing a few extra instructions.  This cost is
+theoretically mitigated or eliminated by reduced instruction cache utilization,
+temporal locality of the stubs, and the stubs' use of MOV instructions over
+PUSH and POP.
+
+This option is not supported with SEH, so it is completely unavailable on
+Windows.  It is also silently disabled if a function:
+
+@enumerate
+@item is built with @option{-mno-sse2} or @option{-fsplit-stack},
+@item has @code{__attribute__ ((ms_hook_prologue))}, or
+@item either throws an exception or explicitly calls @code{__builtin_eh_return}.
+@end enumerate
+
+Support for @option{-fsplit-stack} and @code{__builtin_eh_return} may be
+added at some time in the future, but has not yet been tested.
 
 @item -mtls-dialect=@var{type}
 @opindex mtls-dialect
-- 
2.11.0