Re: [PATCH 7/15] arm: Emit build attributes for PACBTI target feature

[Richard Earnshaw via Gcc-patches]

On 20/10/2022 15:47, Kyrylo Tkachov via Gcc-patches wrote:
Hi Andrea,

-----Original Message-----
From: Gcc-patches <gcc-patches-
bounces+kyrylo.tkachov=arm....@gcc.gnu.org> On Behalf Of Andrea
Corallo via Gcc-patches
Sent: Friday, August 12, 2022 4:31 PM
To: Andrea Corallo via Gcc-patches <gcc-patches@gcc.gnu.org>
Cc: Richard Earnshaw <richard.earns...@arm.com>; nd <n...@arm.com>
Subject: [PATCH 7/15] arm: Emit build attributes for PACBTI target feature

This patch emits assembler directives for PACBTI build attributes as
defined by the
ABI.

<https://github.com/ARM-software/abi-
aa/releases/download/2021Q1/addenda32.pdf>

gcc/ChangeLog:

        * config/arm/arm.c (arm_file_start): Emit EABI attributes for
        Tag_PAC_extension, Tag_BTI_extension, TAG_BTI_use,
TAG_PACRET_use.

gcc/testsuite/ChangeLog:

        * gcc.target/arm/acle/pacbti-m-predef-1.c: New test.
        * gcc.target/arm/acle/pacbti-m-predef-3: Likewise.
        * gcc.target/arm/acle/pacbti-m-predef-6.c: Likewise.
        * gcc.target/arm/acle/pacbti-m-predef-7.c: Likewise.

Co-Authored-By: Tejas Belagod  <tbela...@arm.com>

diff --git a/gcc/config/arm/arm.cc b/gcc/config/arm/arm.cc
index 0068817b0f2..ceec14f84b6 100644
--- a/gcc/config/arm/arm.cc
+++ b/gcc/config/arm/arm.cc
@@ -28349,6 +28349,8 @@ static void
  arm_file_start (void)
  {
    int val;
+  bool pac = (aarch_ra_sign_scope != AARCH_FUNCTION_NONE);
+  bool bti = (aarch_enable_bti == 1);
  
    arm_print_asm_arch_directives
      (asm_out_file, TREE_TARGET_OPTION (target_option_default_node));
@@ -28419,6 +28421,22 @@ arm_file_start (void)
        arm_emit_eabi_attribute ("Tag_ABI_FP_16bit_format", 38,
                             (int) arm_fp16_format);
  
+      if (TARGET_HAVE_PACBTI)
+       {
+         arm_emit_eabi_attribute ("Tag_PAC_extension", 50, 2);
+         arm_emit_eabi_attribute ("Tag_BTI_extension", 52, 2);
+       }
+      else if (pac || bti)
+       {
+         arm_emit_eabi_attribute ("Tag_PAC_extension", 50, 1);
+         arm_emit_eabi_attribute ("Tag_BTI_extension", 52, 1);
+       }

This hunk will set both Tag_PAC_extension and Tag_BTI_extension if only one of 
pac or bti is on. Is that intended?
Would it makes sense to instead set the two Tag_*_extension tags individually 
as in the hunk below?

That's because they are one feature in armv8-m and these tags describe 
the presence of the feature in the architecture.

+
+      if (bti)
+        arm_emit_eabi_attribute ("TAG_BTI_use", 74, 1);
+      if (pac)
+       arm_emit_eabi_attribute ("TAG_PACRET_use", 76, 1);
+

But this describes /use/ by the code of each feature.

R.

        if (arm_lang_output_object_attributes_hook)
        arm_lang_output_object_attributes_hook();
      }

Thanks,
Kyrill