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Subject: Re: [PATCH 4/12] arm: Add testsuite library support for PACBTI target
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To: Andrea Corallo <andrea.corallo@arm.com>,
 Andrea Corallo via Gcc-patches <gcc-patches@gcc.gnu.org>
Cc: Richard Earnshaw <Richard.Earnshaw@arm.com>, nd <nd@arm.com>
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On 28/04/2022 10:40, Andrea Corallo via Gcc-patches wrote:
> Add targeting-checking entities for PACBTI in testsuite
> framework.
> 
> Pre-approved with the requested changes here
> <https://gcc.gnu.org/pipermail/gcc-patches/2021-December/586331.html>.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* testsuite/lib/target-supports.exp:
> 	(check_effective_target_arm_pacbti_hw): New.
> 	* doc/sourcebuild.texi: Document arm_pacbti_hw.
> 
> Co-Authored-By: Tejas Belagod  <tbelagod@arm.com>
> 
+proc check_effective_target_arm_pacbti_hw {} {
+    return [check_runtime arm_pacbti_hw_available {
+	__attribute__ ((naked)) int
+	main (void)
+	{
+	  asm ("pac r12, lr, sp");

So the armv8-m Arm ARM says that this instruction is in the NOP space 
and that it is undefined if we aren't armv8-m.main or higher.

+	  asm ("mov r0, #0");
+	  asm ("autg r12, lr, sp");

This isn't in the nop space, but the Arm ARM says it is unpredictable if 
the extension isn't present.  Unfortunately, that means this isn't a 
particularly reliable way of detecting that the PACBTI feature is present.

However, I can't think off hand of more reliable way of testing this 
since reading the feature register ID_ISAR5 is not possible when in 
unprivileged mode.

So I think we'll have to live with this.

+	  asm ("bx lr");
+	}
+    } ""]

OK.

R.