[PATCH v4 1/2] asan: specify alignment for LASANPC labels

[Ilya Leoshkevich]

On Wed, 2020-07-01 at 21:48 +0200, Ilya Leoshkevich wrote:
> On Wed, 2020-07-01 at 11:57 -0600, Jeff Law wrote:
> > On Wed, 2020-07-01 at 14:29 +0200, Ilya Leoshkevich via Gcc-patches
> > wrote:
> > > gcc/ChangeLog:
> > > 
> > > 2020-06-30  Ilya Leoshkevich  <iii@linux.ibm.com>
> > > 
> > > 	* asan.c (asan_emit_stack_protection): Use CODE_LABEL_BOUNDARY.
> > > 	* defaults.h (CODE_LABEL_BOUNDARY): New macro.
> > > 	* doc/tm.texi: Document CODE_LABEL_BOUNDARY.
> > > 	* doc/tm.texi.in: Likewise.
> > Don't we already have the ability to set label alignments?  See
> > LABEL_ALIGN.
> 
> The following works with -falign-labels=2:
> 
> --- a/gcc/asan.c
> +++ b/gcc/asan.c
> @@ -1524,7 +1524,7 @@ asan_emit_stack_protection (rtx base, rtx
> pbase,
> unsigned int alignb,
>    DECL_INITIAL (decl) = decl;
>    TREE_ASM_WRITTEN (decl) = 1;
>    TREE_ASM_WRITTEN (id) = 1;
> -  SET_DECL_ALIGN (decl, CODE_LABEL_BOUNDARY);
> +  SET_DECL_ALIGN (decl, (1 << LABEL_ALIGN (gen_label_rtx ())) *
> BITS_PER_UNIT);
>    emit_move_insn (mem, expand_normal (build_fold_addr_expr (decl)));
>    shadow_base = expand_binop (Pmode, lshr_optab, base,
>                               gen_int_shift_amount (Pmode,
> ASAN_SHADOW_SHIFT),
> 
> In order to go this way, we would need to raise `-falign-labels=`
> default to 2 for s390, which is not incorrect, but would
> unnecessarily
> clutter asm with `.align 2` before each label.  So IMHO it would be
> nicer to simply ask the backend "what is your target's instruction
> alignment?".

Besides that it would clutter asm with .align 2, another argument
against using LABEL_ALIGN here is that it's semantically different from
what is needed: -falign-labels value, which it returns, is specified by
user for optimization purposes, whereas here we need to query the
architecture's property.

In practical terms, if user specifies -falign-labels=4096, this would
affect how the code is generated here. However, this would be
completely unnecessary: we never jump to decl, its address is only
saved for reporting.

Best regards,
Ilya