[PATCH 2/3] zstd: skip the BMI2 probe when dynamic BMI2 dispatch is disabled

[Usama Arif]

When DYNAMIC_BMI2 is 0 - GCC older than 11, a non-x86 target, or a build
that already has BMI2 on globally - nothing reads the flag.
HUF_compress1X_usingCTable_internal(), FSE_decompress_wksp_bmi2(),
HUF_readStats_body() and the rest all resolve to the default body
without consulting it.  ZSTD_cpuSupportsBmi2() nevertheless issues
CPUID, which on x86 is two serializing instructions, and throws the
answer away.

ZSTD_initDCtx_internal() already wraps its assignment in
#if DYNAMIC_BMI2 - it has to, because the dctx->bmi2 field is itself
declared under that #if.  The two compress-side callers have no such
guard, and ZSTD_CCtx_s::bmi2 is unconditional, so they probe
unconditionally.

Put the test inside ZSTD_cpuSupportsBmi2() so every caller gets it
without having to remember.

Signed-off-by: Usama Arif <usama.arif@xxxxxxxxx>
---
 lib/zstd/common/zstd_internal.h | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/lib/zstd/common/zstd_internal.h b/lib/zstd/common/zstd_internal.h
index 52a79435caf66..41f190b533209 100644
--- a/lib/zstd/common/zstd_internal.h
+++ b/lib/zstd/common/zstd_internal.h
@@ -311,8 +311,13 @@ size_t ZSTD_decodeSeqHeaders(ZSTD_DCtx* dctx, int* nbSeqPtr,
  */
 MEM_STATIC int ZSTD_cpuSupportsBmi2(void)
 {
+#if DYNAMIC_BMI2
     ZSTD_cpuid_t cpuid = ZSTD_cpuid();
     return ZSTD_cpuid_bmi1(cpuid) && ZSTD_cpuid_bmi2(cpuid);
+#else
+    /* Nothing looks at the flag in this configuration. */
+    return 0;
+#endif
 }
 
 #endif   /* ZSTD_CCOMMON_H_MODULE */
-- 
2.53.0-Meta