| Message ID | 20151113165653.GA11709@brightrain.aerifal.cx |
|---|---|
| State | New |
| Headers | show |
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
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
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