disable MEMZERO16X for now on riscV (due to compiler errors)

Author: daanx

include/mimalloc/internal.h

@@ -1523,8 +1523,9 @@ static mi_decl_forceinline void* _mi_memzero_block(mi_block_t* dst, size_t bsize
 
   // fast memzero for small sizes based on overlapping writes (and assuming non-zero size_t-multiple size, and size_t aligned)
   // assumes constant memset(p,0,N) gets optimized to fast simd stores by the compiler
+  // note: disabled on riscv for now as a constant memset is not always replaced correctly by current compilers.
   // (compile with -DMI_USE_MEMZERO16X=0 to disable this)
-  #if !defined(MI_USE_MEMZERO16X) || (MI_USE_MEMZERO16X != 0) // 16x MI_SIZE_SIZE (128 bytes on 64-bit)
+  #if (!defined(MI_USE_MEMZERO16X) && !MI_ARCH_RISCV) || (MI_USE_MEMZERO16X != 0) // 16x MI_SIZE_SIZE (128 bytes on 64-bit)
     if mi_unlikely(bsize < 2*MI_SIZE_SIZE) { // bsize < 16 (8)
       *((size_t*)dst) = 0;
       return dst;

src/alloc.c

@@ -49,7 +49,7 @@ static mi_decl_noinline void mi_page_block_setup_padding(mi_page_t* page, mi_blo
 // Fast allocation in a page: just pop from the free list.
 // Fall back to generic allocation only if the list is empty.
 // Note: even though there is a lot of checks etc in the source,
-// in release mode the (inlined) routine is about 7 instructions with a single test.
+// in release mode the (inlined) routine is about 7 to 10 instructions with a single test.
 static mi_decl_forceinline void* mi_page_malloc_zero(mi_theap_t* theap, mi_page_t* page, size_t size, size_t sample_countdown, bool zero, mi_page_t** ppage) mi_attr_noexcept
 {
   if (page->block_size != 0) { // not the empty theap