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Re: [PATCH 4/10, GAS/ARM] Rework selection of feature bits to base build attributes on


On 21/06/17 11:07, Thomas Preudhomme wrote:
> Hi,
> 
> === Context ===
> 
> This patch is part of a patch series to add support for ARMv8-R
> architecture. Its purpose is to set the feature bits on which to decide
> what the build attributes should be according to the mode
> (autodetection, user specified architecture or CPU, or
> -march/-mcpu=all).
> 
> === Motivation ===
> 
> Currently, the flags variable which is used to determine the build
> attribute is constructed from the instruction used (arm_arch_used and
> thumb_arch_used) as well as the user specified architecture or CPU
> (selected_cpu). This means when several .arch are specified the
> resulting feature bits can be such that no architecture provide them
> all and can thus result in incorrect Tag_CPU_arch. See for instance
> what having both .arch armv8-a and .arch armv8-m.base would result in.
> This is not caught by the testsuite because of further bugs in the
> Tag_CPU_arch build attribute value selection logic (see next patch in
> the series).
> 
> === Patch description ===
> 
> As one would expect, this patch solves the problem by setting flags
> from feature bits used if in autodetection mode [1] and from
> selected_cpu otherwise. The logic to set arm_ext_v1, arm_ext_v4t and
> arm_ext_os feature bits is also moved to only run in autodetection mode
> since otherwise the architecture or CPU would have a consistent set of
> feature bits already.
> 
> [1] No architecture or CPU was specified by the user
> 
> ChangeLog entry is as follows:
> 
> *** gas/ChangeLog ***
> 
> 2017-01-26  Thomas Preud'homme  <thomas.preudhomme@arm.com>
> 
>     * config/tc-arm.c (aeabi_set_public_attributes): Populate flags from
>     feature bits used or selected_cpu depending on whether a CPU was
>     selected by the user.
> 
> === Testing ===
> 
> Testsuite shows no regression when run for arm-none-eabi targets.
> 
> Is this ok for master branch?
> 

OK.


R.
> Best regards,
> 
> Thomas
> 
> 04_rework_feature_bit_selection_logic.patch
> 
> 
> diff --git a/gas/config/tc-arm.c b/gas/config/tc-arm.c
> index 72fd1cc9854428577f1f98ab7703bf4dbdb9cbd5..530a27b1cc915846bcc208878925b60ccd003f6d 100644
> --- a/gas/config/tc-arm.c
> +++ b/gas/config/tc-arm.c
> @@ -26577,19 +26577,33 @@ aeabi_set_public_attributes (void)
>    arm_feature_set arm_arch_v8m_base = ARM_ARCH_V8M_BASE;
>    const cpu_arch_ver_table *p;
>  
> -  /* Choose the architecture based on the capabilities of the requested cpu
> -     (if any) and/or the instructions actually used.  */
> -  ARM_MERGE_FEATURE_SETS (flags, arm_arch_used, thumb_arch_used);
> -  ARM_MERGE_FEATURE_SETS (flags, flags, *mfpu_opt);
> -  ARM_MERGE_FEATURE_SETS (flags, flags, selected_cpu);
> +  /* Autodetection mode, choose the architecture based the instructions
> +     actually used.  */
> +  if (no_cpu_selected ())
> +    {
> +      ARM_MERGE_FEATURE_SETS (flags, arm_arch_used, thumb_arch_used);
>  
> -  if (ARM_CPU_HAS_FEATURE (arm_arch_used, arm_arch_any))
> -    ARM_MERGE_FEATURE_SETS (flags, flags, arm_ext_v1);
> +      if (ARM_CPU_HAS_FEATURE (arm_arch_used, arm_arch_any))
> +	ARM_MERGE_FEATURE_SETS (flags, flags, arm_ext_v1);
>  
> -  if (ARM_CPU_HAS_FEATURE (thumb_arch_used, arm_arch_any))
> -    ARM_MERGE_FEATURE_SETS (flags, flags, arm_ext_v4t);
> +      if (ARM_CPU_HAS_FEATURE (thumb_arch_used, arm_arch_any))
> +	ARM_MERGE_FEATURE_SETS (flags, flags, arm_ext_v4t);
>  
> -  selected_cpu = flags;
> +      /* We need to make sure that the attributes do not identify us as v6S-M
> +	 when the only v6S-M feature in use is the Operating System
> +	 Extensions.  */
> +      if (ARM_CPU_HAS_FEATURE (flags, arm_ext_os))
> +	if (!ARM_CPU_HAS_FEATURE (flags, arm_arch_v6m_only))
> +	  ARM_CLEAR_FEATURE (flags, flags, arm_ext_os);
> +
> +      /* Code run during relaxation relies on selected_cpu being set.  */
> +      selected_cpu = flags;
> +    }
> +  /* Otherwise, choose the architecture based on the capabilities of the
> +     requested cpu.  */
> +  else
> +    flags = selected_cpu;
> +  ARM_MERGE_FEATURE_SETS (flags, flags, *mfpu_opt);
>  
>    /* Allow the user to override the reported architecture.  */
>    if (object_arch)
> @@ -26598,12 +26612,6 @@ aeabi_set_public_attributes (void)
>        ARM_MERGE_FEATURE_SETS (flags, flags, *object_arch);
>      }
>  
> -  /* We need to make sure that the attributes do not identify us as v6S-M
> -     when the only v6S-M feature in use is the Operating System Extensions.  */
> -  if (ARM_CPU_HAS_FEATURE (flags, arm_ext_os))
> -      if (!ARM_CPU_HAS_FEATURE (flags, arm_arch_v6m_only))
> -	ARM_CLEAR_FEATURE (flags, flags, arm_ext_os);
> -
>    tmp = flags;
>    arch = 0;
>    for (p = cpu_arch_ver; p->val; p++)
> 


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