Re: [RFC] [PATCH v3 0/8] [i386] Use out-of-line stubs for ms_abi pro/epilogues

[Daniel Santos <daniel.santos@pobox.com>]

On 02/10/2017 05:34 AM, JonY wrote:
> Hi,
>
> Thanks for the code size reduction patch, I have a few questions:

Thanks for your review!

> 1. How does code compiled with -moutline-msabi-xlogues interact with
> older code compiled without it? Are these only called on ABI transition?

These are only called in functions with the ABI transition from ms_abi 
to sysv_abi.  I'm not completely sure I understand the other question, 
however.  I haven't specifically tested linking two object files, one 
built with and without the option enabled, but I wouldn't expect it to 
produce any problems. Nor have I tested linking object files built with 
-moutline-msabi-xlogues with object files built with an older compiler.

> 2. Does this mean code compiled with -moutline-msabi-xlogues is not
> usable with older GCC? (no a problem, just something to note).

As above, I haven't tested this scenario, but I don't anticipate a 
problem other than general gcc version issues (like the default C++ ABI 
version, etc.). I can set up tests for this, but it will take me a 
little time. I used to have 12 gcc versions installed but I've rebuilt 
my system since then and I only have 4 now.  The nice thing about Gentoo 
is that you build every package yourself.  The bad thing about Gentoo is 
that you build every package yourself.

> 3. Is the old behavior (emit inline code) still default?

Yes.  And I believe that it was for this reason that Sandra (in CC) 
recommended renaming the feature to -m[no-]inline-msabi-xlogues with the 
default to "inline" since "inline" vs "no-inline" is the more common 
language used.  Also, the older behavior is still what is emitted for 
ABI transition functions under various conditions (below), some just 
because I haven't examined them yet.

+    if (!TARGET_SSE)
+      m->outline_ms_sysv = false;
+
+    /* Don't break hot-patched functions.  */
+    else if (ix86_function_ms_hook_prologue (current_function_decl))
+      m->outline_ms_sysv = false;
+
+    /* TODO: Cases not yet examined.  */
+    else if (TARGET_SEH)
+      disable_outline_msabi_xlogues (&warned_seh,
+				     "Structured Exception Handling (SEH)");
+    else if (crtl->calls_eh_return)
+      disable_outline_msabi_xlogues (NULL, "__builtin_eh_return");
+
+    else if (ix86_static_chain_on_stack)
+      disable_outline_msabi_xlogues (NULL, "static call chains");
+
+    else if (ix86_using_red_zone ())
+      disable_outline_msabi_xlogues (NULL, "red zones");
+
+    else if (flag_split_stack)
+      disable_outline_msabi_xlogues (NULL, "split stack");

In fact, DRAP used to be one of these cases and I only got that working with this last version of the patch set.

> mingw-w64 itself does not use any ms_abi/sysv_abi marked functions
> internally, so it should be unaffected. I don't think Cygwin uses any
> either, but I need to double check.

Of course, ms_abi is gcc's default on Windows so it would be sysv_abi 
functions.  I'm *guessing* that just about everything with Cygwin is 
built for Windows, but it would also make sense if (in some odd case) a 
binary built with sysv_abi is used by something and that library or 
program makes the ABI transition when using said hypothetical library.  
Even in these cases, I would not anticipate a problem, although any use 
of SEH would inhibit the optimization.  Of course, I haven't *tested* 
this, so I'm only speaking from what I know. :)

Daniel