[gcc r14-8483] Libatomic: Add checks in ifunc selectors for LSE/LSE2 requirements.

[gcc r14-8483] Libatomic: Add checks in ifunc selectors for LSE/LSE2 requirements.

[Victor Do Nascimento]

https://gcc.gnu.org/g:7dd4466b3954ede754a5a884c5b8fdcd512024d8

commit r14-8483-g7dd4466b3954ede754a5a884c5b8fdcd512024d8
Author: Victor Do Nascimento <victor.donascimento@arm.com>
Date:   Tue Dec 19 23:19:22 2023 +0000

    Libatomic: Add checks in ifunc selectors for LSE/LSE2 requirements.
    
    At present, Evaluation of both `has_lse2(hwcap)' and
    `has_lse128(hwcap)' may require issuing an `mrs' instruction to query
    a system register.  This instruction, when issued from user-space
    results in a trap by the kernel which then returns the value read in
    by the system register.  Given the undesirable nature of the
    computational expense associated with the context switch, it is
    important to implement mechanisms to, wherever possible, forgo the
    operation.
    
    In light of this, given how other architectural requirements serving
    as prerequisites have long been assigned HWCAP bits by the kernel, we
    can inexpensively query for their availability before attempting to
    read any system registers.  Where one of these early tests fail, we
    can assert that the main feature of interest (be it LSE2 or LSE128)
    cannot be present, allowing us to return from the function early and
    skip the unnecessary expensive kernel-mediated access to system
    registers.
    
    libatomic/ChangeLog:
    
            * config/linux/aarch64/host-config.h (has_lse2): Add test for LSE.
            (has_lse128): Add test for LSE2.

Diff:
---
 libatomic/config/linux/aarch64/host-config.h | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/libatomic/config/linux/aarch64/host-config.h b/libatomic/config/linux/aarch64/host-config.h
index 1bc7d8392322..4e3541240633 100644
--- a/libatomic/config/linux/aarch64/host-config.h
+++ b/libatomic/config/linux/aarch64/host-config.h
@@ -64,8 +64,13 @@ typedef struct __ifunc_arg_t {
 static inline bool
 has_lse2 (unsigned long hwcap, const __ifunc_arg_t *features)
 {
+  /* Check for LSE2.  */
   if (hwcap & HWCAP_USCAT)
     return true;
+  /* No point checking further for atomic 128-bit load/store if LSE
+     prerequisite not met.  */
+  if (!(hwcap & HWCAP_ATOMICS))
+    return false;
   if (!(hwcap & HWCAP_CPUID))
     return false;
 
@@ -99,9 +104,11 @@ has_lse128 (unsigned long hwcap, const __ifunc_arg_t *features)
      support in older kernels as it is of CPU feature absence.  Try fallback
      method to guarantee LSE128 is not implemented.
 
-     In the absence of HWCAP_CPUID, we are unable to check for LSE128.  */
-  if (!(hwcap & HWCAP_CPUID))
-    return false;
+     In the absence of HWCAP_CPUID, we are unable to check for LSE128.
+     If feature check available, check LSE2 prerequisite before proceeding.  */
+  if (!(hwcap & HWCAP_CPUID) || !(hwcap & HWCAP_USCAT))
+     return false;
+
   unsigned long isar0;
   asm volatile ("mrs %0, ID_AA64ISAR0_EL1" : "=r" (isar0));
   if (AT_FEAT_FIELD (isar0) >= 3)