[gcc r17-3399] libcpp: require SVE as well as SVE2 for the AArch64 lexer [PR126935]

https://gcc.gnu.org/g:ec2890c334eaccbe37e2e2eae6a4de62fefdcbf8

commit r17-3399-gec2890c334eaccbe37e2e2eae6a4de62fefdcbf8
Author: Kyrylo Tkachov <[email protected]>
Date:   Wed Aug 19 07:14:10 2026 +0200

    libcpp: require SVE as well as SVE2 for the AArch64 lexer [PR126935]
    
    init_vectorized_lexer only tested HWCAP2_SVE2.  Some virtual machines
    advertise SVE2 while leaving SVE clear, which is a nonsense
    configuration but one that occurs in practice, and there the compiler
    dies as soon as it lexes anything:
    
      build/genmatch --gimple ...
      make[3]: *** [Makefile:3102: s-gimple-match] Illegal instruction
    
    Test HWCAP_SVE alongside HWCAP2_SVE2 so the run-time dispatch declines
    the SVE2 helper on such a machine and keeps using Neon.
    SVE2 without SVE is a nonsense configuration architecture-wise and
    arguably the VM hypervisor is at fault here, but it is easy to work around
    for the sake of fixing bootstrap on such machines.
    
    libcpp/ChangeLog:
    
            PR target/126935
            * lex.cc (HWCAP_SVE): Define if not already defined.
            (init_vectorized_lexer): Require HWCAP_SVE as well as
            HWCAP2_SVE2.
    
    Signed-off-by: Kyrylo Tkachov <[email protected]>

Diff:
---
 libcpp/lex.cc | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/libcpp/lex.cc b/libcpp/lex.cc
index ee6d6e01803d..bd35e28ce8f4 100644
--- a/libcpp/lex.cc
+++ b/libcpp/lex.cc
@@ -740,6 +740,9 @@ done:
 #include <arm_sve.h>
 #include <sys/auxv.h>
 
+#ifndef HWCAP_SVE
+#define HWCAP_SVE (1 << 22)
+#endif
 #ifndef HWCAP2_SVE2
 #define HWCAP2_SVE2 (1 << 1)
 #endif
@@ -810,7 +813,11 @@ static bool lexer_has_sve2;
 static inline void
 init_vectorized_lexer (void)
 {
-  lexer_has_sve2 = (getauxval (AT_HWCAP2) & HWCAP2_SVE2) != 0;
+  /* MATCH aside, the scanner is built from base SVE instructions, so both
+     features have to be present.  Some virtual machines advertise SVE2
+     without SVE, and there even the leading CNTB traps.  */
+  lexer_has_sve2 = ((getauxval (AT_HWCAP) & HWCAP_SVE) != 0
+		    && (getauxval (AT_HWCAP2) & HWCAP2_SVE2) != 0);
 }
 
 #endif