Fix AltiVec bool keyword conflict with wrapper header

GCC's AltiVec extension makes 'bool' a keyword meaning '__vector __bool
int' when compiling with -maltivec. This conflicts with C99/C11 stdbool.h
where bool means _Bool, breaking all scalar bool usage in HACL* BLAKE2
SIMD128 code.

Note: the simpler -Dbool=_Bool approach does not work because altivec.h
re-enables the keyword after the macro is defined.

The fix is a small wrapper header (ppc_altivec_fix.h) that:
1. Includes altivec.h (which activates the bool keyword)
2. Immediately #undefs bool/true/false
3. Redefines them as C99 _Bool/1/0

This header is force-included (-include) via LIBHACL_SIMD128_FLAGS
before HACL source files. The __ALTIVEC__ guard ensures it only
activates on PowerPC. Vector boolean types remain available via
the explicit __vector __bool syntax.

This is a known GCC/AltiVec interaction; the same approach is used
by FFmpeg and other projects that mix AltiVec intrinsics with C99.

Verified: Hacl_Hash_Blake2s_Simd128.c compiles cleanly on POWER8
(GCC 10.5, -maltivec -mvsx -std=c11) producing a valid ELF64 object.

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

Author: Scottcjn

Modules/_hacl/ppc_altivec_fix.h

@@ -0,0 +1,32 @@
+/*
+ * PowerPC AltiVec bool keyword fix for HACL* BLAKE2 SIMD128.
+ *
+ * When GCC compiles with -maltivec, it makes "bool" a keyword meaning
+ * "__vector __bool int" (a 128-bit vector type). This conflicts with
+ * C99/C11 stdbool.h where bool means _Bool (a scalar type).
+ *
+ * The -Dbool=_Bool workaround does NOT work because altivec.h re-enables
+ * the keyword after the macro is defined. Instead, this header includes
+ * altivec.h first, then undefines the bool/true/false keywords and
+ * restores the C99 scalar definitions. Vector boolean types remain
+ * accessible via the explicit __vector __bool syntax.
+ *
+ * This header is force-included (-include) before HACL SIMD128 sources
+ * via LIBHACL_SIMD128_FLAGS in configure.ac.
+ */
+#ifndef PPC_ALTIVEC_BOOL_FIX_H
+#define PPC_ALTIVEC_BOOL_FIX_H
+
+#if defined(__ALTIVEC__) || defined(__VSX__)
+#include <altivec.h>
+
+#undef bool
+#undef true
+#undef false
+
+#define bool _Bool
+#define true 1
+#define false 0
+#endif /* __ALTIVEC__ || __VSX__ */
+
+#endif /* PPC_ALTIVEC_BOOL_FIX_H */

configure

@@ -8133,7 +8133,7 @@ then
     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 -flax-vector-conversions -Wno-return-type"]
+      [LIBHACL_SIMD128_FLAGS="-maltivec -mvsx -flax-vector-conversions -Wno-return-type -include \$(srcdir)/Modules/_hacl/ppc_altivec_fix.h"]
 
       AC_DEFINE([_Py_HACL_CAN_COMPILE_VEC128], [1], [
         HACL* library can compile SIMD128 implementations])