gh-138114: Enable HACL BLAKE2 SIMD128 vectorization on PowerPC64

The HACL* library's libintvector.h already contains a complete
PowerPC64 AltiVec/VSX implementation of vec128 operations (lines
800-926), but CPython's configure never enables it because the
SIMD128 detection only checks for x86 SSE.

This adds PowerPC64 detection as a fallback in the SSE check's
else-branch of configure.ac, testing for -maltivec -mvsx compiler
flags, which enables SIMD-accelerated BLAKE2s hashing on POWER8+.

This implements the TODO at configure.ac line 8113:
"This can be extended here to detect e.g. Power8, which HACL*
should also support."

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Scottcjn

Misc/NEWS.d/next/Build/2026-03-18-00-00-00.gh-issue-138114.PwrVec.rst

@@ -0,0 +1,3 @@
+Enable HACL* BLAKE2s SIMD128 vectorization on PowerPC64 (POWER8+) using
+AltiVec/VSX instructions. The HACL* library already contained a complete
+PowerPC64 vec128 implementation but CPython's configure never enabled it.

configure

@@ -8110,7 +8110,7 @@ fi
 if test "$ac_sys_system" != "Linux-android" -a "$ac_sys_system" != "WASI" || \
    { test -n "$ANDROID_API_LEVEL" && test "$ANDROID_API_LEVEL" -ge 28; }
 then
-  dnl This can be extended here to detect e.g. Power8, which HACL* should also support.
+  dnl Check for x86 SSE support (SIMD128)
   AX_CHECK_COMPILE_FLAG([-msse -msse2 -msse3 -msse4.1 -msse4.2],[
     [LIBHACL_SIMD128_FLAGS="-msse -msse2 -msse3 -msse4.1 -msse4.2"]
 
@@ -8129,7 +8129,20 @@ then
       AC_MSG_RESULT([standard])
     fi
 
-  ], [], [-Werror])
+  ], [
+    dnl x86 SSE not available; check for PowerPC AltiVec/VSX (Power8+).
+    dnl HACL* libintvector.h has a complete vec128 implementation for __powerpc64__.
+    AX_CHECK_COMPILE_FLAG([-maltivec -mvsx],[
+      [LIBHACL_SIMD128_FLAGS="-maltivec -mvsx"]
+
+      AC_DEFINE([_Py_HACL_CAN_COMPILE_VEC128], [1], [
+        HACL* library can compile SIMD128 implementations])
+
+      [LIBHACL_BLAKE2_SIMD128_OBJS="Modules/_hacl/Hacl_Hash_Blake2s_Simd128.o"]
+      AC_MSG_CHECKING([for HACL* SIMD128 implementation])
+      AC_MSG_RESULT([PowerPC AltiVec/VSX])
+    ], [], [-Werror])
+  ], [-Werror])
 fi
 AC_SUBST([LIBHACL_SIMD128_FLAGS])
 AC_SUBST([LIBHACL_BLAKE2_SIMD128_OBJS])