32-bit x86 CPUID code returns EBP instead of EBX

[bit4]

The inline assembly used by 32-bit x86 code needs to save and restore the EBX register because position independent code needs it unfortunately (https://ewontfix.com/18/). Since the CPUID instruction also uses the EBX register to return information, the inline assembly has to copy that data into another register (ESI in this case) before it restores the original EBX.

However, both the cpuid() and the cpuidex() functions contain a typo, and they return whatever is in the EBP register (essentially a random value) instead of EBX. This breaks run-time detection of AVX2 and newer CPU features.

The fix is very simple:

diff --git a/c/blake3_dispatch.c b/c/blake3_dispatch.c
index 7daf43e..b97b7c0 100644
--- a/c/blake3_dispatch.c
+++ b/c/blake3_dispatch.c
@@ -97,7 +97,7 @@ static void cpuid(uint32_t out[4], uint32_t id) {
   __cpuid((int *)out, id);
 #else
 #if defined(__i386__) || defined(_M_IX86)
-  __asm__ __volatile__("pushl %%ebx\ncpuid\nmovl %%ebp, %%esi\npopl %%ebx"
+  __asm__ __volatile__("pushl %%ebx\ncpuid\nmovl %%ebx, %%esi\npopl %%ebx"
                        : "=a"(out[0]), "=S"(out[1]), "=c"(out[2]), "=d"(out[3])
                        : "a"(id));
 #else
@@ -113,7 +113,7 @@ static void cpuidex(uint32_t out[4], uint32_t id, uint32_t sid) {
   __cpuidex((int *)out, id, sid);
 #else
 #if defined(__i386__) || defined(_M_IX86)
-  __asm__ __volatile__("pushl %%ebx\ncpuid\nmovl %%ebp, %%esi\npopl %%ebx"
+  __asm__ __volatile__("pushl %%ebx\ncpuid\nmovl %%ebx, %%esi\npopl %%ebx"
                        : "=a"(out[0]), "=S"(out[1]), "=c"(out[2]), "=d"(out[3])
                        : "a"(id), "c"(sid));
 #else

[sneves]

Nicely spotted!

The fix is a bit more involved that what you suggested; I used xchg instead of a push-pop sequence.