Re: master: Build failure in malloc with GCC 7

[Florian Weimer <fweimer at redhat dot com>]

On 07/12/2017 02:29 PM, Andreas Schwab wrote:
> On Jul 12 2017, Florian Weimer <fweimer@redhat.com> wrote:
> 
>> The attached patch adds an assert which reveals to the GCC optimizers
>> that the global_max_fast can never MAX_FAST_SIZE, so this particular
>> issue goes away.  However, there is a cost in terms of code size because
>> it affects many places in malloc.
> 
> Can __builtin_unreachable avoid the runtime overhead?

Interesting idea.  I have never used __builtin_unreachable before, I think.

Current master:

   text    data     bss     dec     hex filename
  64592    2392     104   67088   10610 /root/build/malloc/malloc.o

My second patch:

   text    data     bss     dec     hex filename
  64424    2392     104   66920   10568 /root/build/malloc/malloc.o

Attached patch:

   text    data     bss     dec     hex filename
  64504    2392     104   67000   105b8 /root/build/malloc/malloc.o

Looking at the GIMPLE dumps, the code is somewhat improved over my
second patch.

Thanks,
Florian

diff --git a/malloc/malloc.c b/malloc/malloc.c
index 54e406b..da7876a 100644
--- a/malloc/malloc.c
+++ b/malloc/malloc.c
@@ -1658,6 +1658,9 @@ typedef struct malloc_chunk *mfastbinptr;
 #define arena_is_corrupt(A)	(((A)->flags & ARENA_CORRUPTION_BIT))
 #define set_arena_corrupt(A)	((A)->flags |= ARENA_CORRUPTION_BIT)
 
+/* Maximum size of memory handled in fastbins.  */
+static INTERNAL_SIZE_T global_max_fast;
+
 /*
    Set value of max_fast.
    Use impossibly small value if 0.
@@ -1668,8 +1671,20 @@ typedef struct malloc_chunk *mfastbinptr;
 #define set_max_fast(s) \
   global_max_fast = (((s) == 0)						      \
                      ? SMALLBIN_WIDTH : ((s + SIZE_SZ) & ~MALLOC_ALIGN_MASK))
-#define get_max_fast() global_max_fast
 
+static inline INTERNAL_SIZE_T
+get_max_fast (void)
+{
+  /* Tell the glibc optimizers that global_max_fast is never larger
+     than MAX_FAST_SIZE.  This avoids out-of-bounds array accesses in
+     _int_malloc after constant propagation of the size parameter.
+     (The code never executes because malloc preserves the
+     global_max_fast invariant, but the optimizers may not recognize
+     this.)  */
+  if (global_max_fast > MAX_FAST_SIZE)
+    __builtin_unreachable ();
+  return global_max_fast;
+}
 
 /*
    ----------- Internal state representation and initialization -----------
@@ -1797,9 +1812,6 @@ static struct malloc_par mp_ =
 #endif
 };
 
-/* Maximum size of memory handled in fastbins.  */
-static INTERNAL_SIZE_T global_max_fast;
-
 /*
    Initialize a malloc_state struct.