F_MALLOC: Add LHS coalescing on free() operations

This is a continuation/rework of commit fb9a377, which intended to
address the de-fragmentation issues where performance would drop.
Unfortunately, there were still memory allocation / releasing patterns
which still lead to heavy fragmentation.  For example, an MI "dlg_list"
command, which allocates fragments from Left -> Right as the dialog list
is walked:

    Chunk-1  Chunk-2  Chunk-3 ... Chunk-N

After building the MI output, the chunks were also freed Left -> Right.
Due to the former allocator logic of RHS-only coalescing, the coalescing
would never happen!  In some cases, this could lead to fragmentation of
an indefinite amount of memory, given enough time (e.g., even 8 GB PKG).

This patch adds LHS coalescing to F_MALLOC, such that when "Chunk-2" is
freed, in the above example, the allocator tries to merge it with both
"Chunk-1" and "Chunk-3" before exiting the free() function (depending on
the state of chunks 1 and 3, this could mean 0, 1 or 2 coalescings).

This F_MALLOC patch was stress-tested using the mem/test/ testing suite.

Many thanks to @fedkis and @ankogan for helping troubleshoot the issue
and also test the current version of this patch!
Fixes #2726

(cherry picked from commit bdaaf60)

mem/f_malloc.c

@@ -26,6 +26,7 @@
 #include <string.h>
 #include <stdio.h>
 #include <stdlib.h>
+#include <assert.h>
 
 #include "f_malloc.h"
 #include "../dprint.h"
@@ -40,6 +41,7 @@
 #define FRAG_OVERHEAD	(sizeof(struct fm_frag))
 #define frag_is_free(_f) ((_f)->prev)
 
+#define FRAG_PREV(f) ((f)->pf)
 #define FRAG_NEXT(f) \
 	((struct fm_frag *)((char *)(f) + sizeof(struct fm_frag) + (f)->size))
 
@@ -216,10 +218,14 @@ struct fm_block *fm_malloc_init(char *address, unsigned long size, char *name)
 	/* init initial fragment*/
 	fm->first_frag->size=size-init_overhead;
 	fm->last_frag->size=0;
+	fm->last_frag->pf=fm->first_frag;
 
 	fm->last_frag->prev=NULL;
 	fm->first_frag->prev=NULL;
 
+	assert(((char *)fm->first_frag + sizeof *fm->first_frag + fm->first_frag->size)
+				== (char *)fm->last_frag);
+
 	/* link initial fragment into the free list*/
 
 	fm_insert_free(fm, fm->first_frag);

mem/f_malloc.h

@@ -64,6 +64,9 @@ struct fm_frag {
 		long reserved;
 	} u;
 	struct fm_frag **prev;
+
+	struct fm_frag *pf; /* previous "physical" frag */
+
 #ifdef DBG_MALLOC
 	const char *file;
 	const char *func;

mem/f_malloc_dyn.h

@@ -58,6 +58,8 @@ void fm_split_frag(struct fm_block *fm, struct fm_frag *frag,
 		/*split the fragment*/
 		n=FRAG_NEXT(frag);
 		n->size=rest-FRAG_OVERHEAD;
+		n->pf=frag;
+		FRAG_NEXT(n)->pf=n;
 
 		/*
 		 * The real used memory does not increase, as the frag memory is not
@@ -217,7 +219,7 @@ void fm_free(struct fm_block *fm, void *p, const char *file,
              const char *func, unsigned int line)
 #endif
 {
-	struct fm_frag *f, *n;
+	struct fm_frag *f, *neigh;
 
 	#ifdef DBG_MALLOC
 	LM_GEN1(memlog, "%s_free(%p), called from %s: %s(%d)\n", fm->name, p, file,
@@ -242,18 +244,34 @@ void fm_free(struct fm_block *fm, void *p, const char *file,
 #endif
 
 	/* attempt to join with a next fragment that also happens to be free */
-	n = FRAG_NEXT(f);
-	if (((char*)n < (char*)fm->last_frag) &&  frag_is_free(n)) {
-		fm_remove_free(fm, n);
+	neigh = FRAG_NEXT(f);
+	if (((char*)neigh < (char*)fm->last_frag) &&  frag_is_free(neigh)) {
+		fm_remove_free(fm, neigh);
 		/* join */
-		f->size += n->size + FRAG_OVERHEAD;
+		f->size += neigh->size + FRAG_OVERHEAD;
+		FRAG_NEXT(neigh)->pf = f;
 
 		#if defined(DBG_MALLOC) || defined(STATISTICS)
 		//fm->real_used -= FRAG_OVERHEAD;
 		fm->used += FRAG_OVERHEAD;
 		#endif
 	}
 
+	/* attempt to join with a prev fragment that also happens to be free */
+	neigh = FRAG_PREV(f);
+	if (neigh && frag_is_free(neigh)) {
+		fm_remove_free(fm, neigh);
+		neigh->size += f->size + FRAG_OVERHEAD;
+		FRAG_NEXT(f)->pf = neigh;
+
+		#if defined(DBG_MALLOC) || defined(STATISTICS)
+		//fm->real_used -= FRAG_OVERHEAD;
+		fm->used += FRAG_OVERHEAD;
+		#endif
+
+		f = neigh;
+	}
+
 #ifdef DBG_MALLOC
 	f->file = file;
 	f->func = func;
@@ -354,6 +372,7 @@ void *fm_realloc(struct fm_block *fm, void *p, unsigned long size,
 			fm_remove_free(fm,n);
 			/* join */
 			f->size += n->size + FRAG_OVERHEAD;
+			FRAG_NEXT(f)->pf = f;
 
 			#if defined(DBG_MALLOC) || defined(STATISTICS)
 			//fm->real_used -= FRAG_OVERHEAD;