Re: [PATCH v2] x86-64: Update _dl_tlsdesc_dynamic to preserve vector registers

[Florian Weimer <fweimer@redhat.com>]

* H. J. Lu:

> Compiler generates the following instruction sequence for GNU2 dynamic
> TLS access:
>
> 	leaq	tls_var@TLSDESC(%rip), %rax
> 	call	*tls_var@TLSCALL(%rax)
>
> CALL instruction may be transparent to compiler which assumes all
> registers, except for RAX, are unchanged after CALL.  At run-time,
> _dl_tlsdesc_dynamic is called, which calls __tls_get_addr on the
> slow path.  __tls_get_addr is a normal function which doesn't
> preserve any caller-saved registers.  _dl_tlsdesc_dynamic saves and
> restores integer caller-saved registers, but doesn't preserve any
> vector registers which are caller-saved.  Add _dl_tlsdesc_dynamic
> IFUNC functions for FXSAVE, XSAVE and XSAVEC to save and restore
> all vector registers.  This fixes BZ #31372.

What about the flags register?  It's still clobbered.

What about stack pointer alignment?

> +	/* Besides rdi and rsi, saved above, save rcx, rdx, r8, r9,
> +	   r10 and r11.  */

I would prefer to see some explicit ABI action first before making such
decisions.  Maybe %r11 could be a scratch register available for use by
the TLSDESC call?  Or does current GCC already assume that all scalar
registers are preserved?

The other way to fix this is to preallocate everything, so that the need
for the slow path goes away.

Thanks,
Florian