commit dd9ced1c6d7ae7956b1b667b6289c143c2070255
author Xi Ruoyao <xry111@xry111.site> Sat Jul 19 14:36:11 2025 +0800
committer Frank Barchard <fbarchard@chromium.org> Thu Jul 24 23:43:54 2025 -0700
tree 1b20536fed742243085cbaa6a8644860939bb1ba
parent 96134e95a7468af63ff2ea74a85b0d6db2d2c25e

loong64: Use HWCAP instead of CPUCFG to detect LSX/LASX

Per the Software Development and Build Convention for LoongArch™
Architectures manual, on Linux we should use HWCAP instead of CPUCFG to
detect if LSX/LASX is available.  The reason is the kernel may be
configured to disable them, and CPUCFG cannot provide info about the
kernel support.

Change-Id: I3f1b23e6d4c91c7da81311fbbe294e36ff178121
Reviewed-on: https://chromium-review.googlesource.com/c/libyuv/libyuv/+/6772567
Reviewed-by: Frank Barchard <fbarchard@chromium.org>
Reviewed-by: Wan-Teh Chang <wtc@google.com>

source/cpu_id.cc

diff --git a/source/cpu_id.cc b/source/cpu_id.cc
index 0e0740b..95a6586 100644
--- a/source/cpu_id.cc
+++ b/source/cpu_id.cc
@@ -23,7 +23,7 @@
 #include <stdio.h>  // For fopen()
 #include <string.h>
 
-#if defined(__linux__) && defined(__aarch64__)
+#if defined(__linux__) && (defined(__aarch64__) || defined(__loongarch__))
 #include <sys/auxv.h>  // For getauxval()
 #endif
 
@@ -388,21 +388,20 @@
   return flag;
 }
 
-#define LOONGARCH_CFG2 0x2
-#define LOONGARCH_CFG2_LSX (1 << 6)
-#define LOONGARCH_CFG2_LASX (1 << 7)
+#if defined(__loongarch__) && defined(__linux__)
+// Define hwcap values ourselves: building with an old auxv header where these
+// hwcap values are not defined should not prevent features from being enabled.
+#define YUV_LOONGARCH_HWCAP_LSX (1 << 4)
+#define YUV_LOONGARCH_HWCAP_LASX (1 << 5)
 
-#if defined(__loongarch__)
 LIBYUV_API SAFEBUFFERS int LoongarchCpuCaps(void) {
   int flag = 0;
-  uint32_t cfg2 = 0;
+  unsigned long hwcap = getauxval(AT_HWCAP);
 
-  __asm__ volatile("cpucfg %0, %1 \n\t" : "+&r"(cfg2) : "r"(LOONGARCH_CFG2));
-
-  if (cfg2 & LOONGARCH_CFG2_LSX)
+  if (hwcap & YUV_LOONGARCH_HWCAP_LSX)
     flag |= kCpuHasLSX;
 
-  if (cfg2 & LOONGARCH_CFG2_LASX)
+  if (hwcap & YUV_LOONGARCH_HWCAP_LASX)
     flag |= kCpuHasLASX;
   return flag;
 }