[2.29 COMMITTED] Base max_fast on alignment, not width, of bins (Bug 24903)

[2.29 COMMITTED] Base max_fast on alignment, not width, of bins (Bug 24903)

[Arjun Shankar]

set_max_fast sets the "impossibly small" value based on,
eventually, MALLOC_ALIGNMENT.  The comparisons for the smallest
chunk used is, eventually, MIN_CHUNK_SIZE.  Note that i386
is the only platform where these are the same, so a smallest
chunk *would* be put in a no-fastbins fastbin.

This change calculates the "impossibly small" value
based on MIN_CHUNK_SIZE instead, so that we can know it will
always be impossibly small.

(cherry picked from commit ff12e0fb91b9072800f031cb21fb2651ee7b6251)
---
 malloc/malloc.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/malloc/malloc.c b/malloc/malloc.c
index ecdb2a0f36..9ce0941b9a 100644
--- a/malloc/malloc.c
+++ b/malloc/malloc.c
@@ -1635,7 +1635,7 @@ static INTERNAL_SIZE_T global_max_fast;
 
 #define set_max_fast(s) \
   global_max_fast = (((s) == 0)						      \
-                     ? SMALLBIN_WIDTH : ((s + SIZE_SZ) & ~MALLOC_ALIGN_MASK))
+                     ? MIN_CHUNK_SIZE / 2 : ((s + SIZE_SZ) & ~MALLOC_ALIGN_MASK))
 
 static inline INTERNAL_SIZE_T
 get_max_fast (void)
-- 
2.21.0