[PATCH] Fix SH/FDPIC bad codegen with ssp enabled

[PATCH] Fix SH/FDPIC bad codegen with ssp enabled

[Rich Felker]

The "chk_guard_add" pattern used for loading the GOT slot address for
__stack_chk_guard hard-codes use of r12 as a fixed GOT register and
thus is not suitable for FDPIC, where the saved initial value of r12
from function entry is what we need.

I would actually prefer removing this hack entirely if possible. I
tried non-FDPIC with it disabled and did not experience any problems;
I suspect it was written to work around a bug that no longer exists.

2015-11-13  Rich Felker <dalias@libc.org>

	gcc/
	* config/sh/sh.md (symGOT_load): Suppress __stack_chk_guard
	address loading hack for FDPIC targets.

diff --git a/gcc/config/sh/sh.md b/gcc/config/sh/sh.md
index 7a40d0f..45c9995 100644
--- a/gcc/config/sh/sh.md
+++ b/gcc/config/sh/sh.md
@@ -11082,7 +11082,7 @@ (define_expand "symGOT_load"
   operands[2] = !can_create_pseudo_p () ? operands[0] : gen_reg_rtx (Pmode);
   operands[3] = !can_create_pseudo_p () ? operands[0] : gen_reg_rtx (Pmode);
 
-  if (!TARGET_SHMEDIA
+  if (!TARGET_SHMEDIA && !TARGET_FDPIC
       && flag_stack_protect
       && GET_CODE (operands[1]) == CONST
       && GET_CODE (XEXP (operands[1], 0)) == UNSPEC

Re: [PATCH] Fix SH/FDPIC bad codegen with ssp enabled

[Kaz Kojima]

Rich Felker <dalias@libc.org> wrote:
> The "chk_guard_add" pattern used for loading the GOT slot address for
> __stack_chk_guard hard-codes use of r12 as a fixed GOT register and
> thus is not suitable for FDPIC, where the saved initial value of r12
> from function entry is what we need.

The patch is OK.  Committed as revision 230366.

> I would actually prefer removing this hack entirely if possible. I
> tried non-FDPIC with it disabled and did not experience any problems;
> I suspect it was written to work around a bug that no longer exists.

Even we don't see the problem without that, it'll be a latent
issue with the old reload, I think.  When SH switches to LRA
completely, this should be revisit.

Regards,
	kaz

Re: [PATCH] Fix SH/FDPIC bad codegen with ssp enabled

[Rich Felker]

On Sat, Nov 14, 2015 at 09:24:32AM +0900, Kaz Kojima wrote:
> Rich Felker <dalias@libc.org> wrote:
> > The "chk_guard_add" pattern used for loading the GOT slot address for
> > __stack_chk_guard hard-codes use of r12 as a fixed GOT register and
> > thus is not suitable for FDPIC, where the saved initial value of r12
> > from function entry is what we need.
> 
> The patch is OK.  Committed as revision 230366.

Thanks!

> > I would actually prefer removing this hack entirely if possible. I
> > tried non-FDPIC with it disabled and did not experience any problems;
> > I suspect it was written to work around a bug that no longer exists.
> 
> Even we don't see the problem without that, it'll be a latent
> issue with the old reload, I think.  When SH switches to LRA
> completely, this should be revisit.

I thought about trying a patch to eliminate the use of the hard-coded
PIC register and instead generate a pseudo for the GOT address. Then
some FDPIC and non-FDPIC code could potentially be unified to use this
pseudo, just with different ways of defining it (r12-at-entry vs
PC-relative symbolic load of _GLOBAL_OFFSET_TABLE_). Would this be
acceptable?

I've already done some experiments removing r12 from fixed_regs for
FDPIC and nothing visibly broke; gcc correctly used r12 as a temp in
functions which didn't need the GOT address, and codegen was improved
even in functions which do use it. These results were observed on musl
libc.so.

Rich