Date: Tue, 08 Jan 2013 10:22:17 +0100 From: Dimitry Andric <dim@FreeBSD.org> To: David Chisnall <theraven@freebsd.org> Cc: Stefan Farfeleder <stefanf@freebsd.org>, Nathan Whitehorn <nwhitehorn@freebsd.org>, freebsd-current@freebsd.org Subject: Re: clang 3.2 RC2 miscompiles libgcc? Message-ID: <50EBE549.9070808@FreeBSD.org> In-Reply-To: <1D415F08-5FFC-41B5-9A0D-E03757800E0C@freebsd.org> References: <20121227150724.GA1431@mole.fafoe.narf.at> <50DC65F5.6060004@freebsd.org> <50E0BD66.4070609@FreeBSD.org> <20130102135950.GA1464@mole.fafoe.narf.at> <20130104154940.GD1430@mole.fafoe.narf.at> <20130106141708.GA1418@mole.fafoe.narf.at> <50E9916F.3040500@FreeBSD.org> <20130106160331.GB1418@mole.fafoe.narf.at> <50EB5868.2050509@FreeBSD.org> <1D415F08-5FFC-41B5-9A0D-E03757800E0C@freebsd.org>
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On 2013-01-08 09:08, David Chisnall wrote:
> On 7 Jan 2013, at 23:21, Dimitry Andric wrote:
>> This is at least the direction I'm looking at. It seems that in some
>> cases with __builtin_eh_return(), llvm does not see that registers can
>> be clobbered, and it doesn't save and restore them.
> Do you mean that some registers were clobbered by a prior call? __builtin_eh_return() doesn't return, so whether it clobbers anything or not isn't something that should matter. The preceding call is __builtin_frob_return_addr, which seems to be a no-op, so it shouldn't clobber any registers either...
No, I mean that gcc seems to take great care in saving and restoring
almost all important registers in a function, if that function contains
a call to __builtin_eh_return.
If you look at expand_eh_return() in contrib/gcc/except.c, you can see
that it sets the special variable 'current_function_calls_eh_return'.
This influences the code generation all over the place, and specifically
the saving of registers in contrib/gcc/config/i386/i386.c:
======================================================================
/* Return 1 if we need to save REGNO. */
static int
ix86_save_reg (unsigned int regno, int maybe_eh_return)
{
if (pic_offset_table_rtx
&& regno == REAL_PIC_OFFSET_TABLE_REGNUM
&& (regs_ever_live[REAL_PIC_OFFSET_TABLE_REGNUM]
|| current_function_profile
|| current_function_calls_eh_return
|| current_function_uses_const_pool))
{
if (ix86_select_alt_pic_regnum () != INVALID_REGNUM)
return 0;
return 1;
}
if (current_function_calls_eh_return && maybe_eh_return)
{
unsigned i;
for (i = 0; ; i++)
{
unsigned test = EH_RETURN_DATA_REGNO (i);
if (test == INVALID_REGNUM)
break;
if (test == regno)
return 1;
}
}
[...]
/* Emit code to save registers in the prologue. */
static void
ix86_emit_save_regs (void)
{
unsigned int regno;
rtx insn;
for (regno = FIRST_PSEUDO_REGISTER; regno-- > 0; )
if (ix86_save_reg (regno, true))
{
insn = emit_insn (gen_push (gen_rtx_REG (Pmode, regno)));
RTX_FRAME_RELATED_P (insn) = 1;
}
}
======================================================================
On i386, most registers are touched anyway in _Unwind_Resume, so clang
will already save and restore them. But on amd64, there are more
registers than local variables, so clang only seems to save a few; not
enough, in any case. This is why I added the asm statement which
clobbers all those registers, forcing clang to save and restore them.
This fixes most of the crashes I was able to reproduce. I think I still
have another unrelated issue in libgcc with clang, but this only occurs
when compiling the testcases with gcc 4.7, and very high optimization.
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