GCC: disable default heap limit checking in sbrk

- __HeapLimit symbol was used in sbrk implementation to check for out of heap condition.
  In many cases linker scripts do not provide this symbol, hence by default this check
  is now disabled.

- One can re-enable heap limit check by using define SBRK_HEAP_LIMIT_CHECK and provide
  symbol __HeapLimit which designates the end of heap section.

Author: VladimirUmek

source/gcc/retarget_syscalls.c

@@ -46,13 +46,16 @@
 #endif
 
 #ifndef STDIN_ECHO
-#define STDIN_ECHO      0       /* STDIN: echo to STDOUT */
+#define STDIN_ECHO              0 /* STDIN: echo to STDOUT */
 #endif
 #ifndef STDOUT_CR_LF
-#define STDOUT_CR_LF    0       /* STDOUT: add CR for LF */
+#define STDOUT_CR_LF            0 /* STDOUT: add CR for LF */
 #endif
 #ifndef STDERR_CR_LF
-#define STDERR_CR_LF    0       /* STDERR: add CR for LF */
+#define STDERR_CR_LF            0 /* STDERR: add CR for LF */
+#endif
+#ifndef SBRK_HEAP_LIMIT_CHECK
+#define SBRK_HEAP_LIMIT_CHECK   0 /* SBRK: use __HeapLimit symbol to check for out of heap condition */
 #endif
 
 /* Forward prototypes. */
@@ -613,11 +616,12 @@ pid_t _wait (int *stat_loc) {
 /* Extend heap space by incr bytes (reentrant version) */
 __attribute__((weak))
 void *_sbrk_r (struct _reent *reent, ptrdiff_t incr) {
+  #if (SBRK_HEAP_LIMIT_CHECK != 0)
   extern char  __HeapLimit;
+  #endif
   extern char  __HeapStart __ASM("end");
   static char *heap;
          char *heap_prev;
-         void *p;
 
   if (heap == NULL) {
     /* Initialize current heap memory address */
@@ -626,18 +630,16 @@ void *_sbrk_r (struct _reent *reent, ptrdiff_t incr) {
 
   heap_prev = heap;
 
+  #if (SBRK_HEAP_LIMIT_CHECK != 0)
   if ((heap + incr) > &__HeapLimit) {
     /* Out of heap memory */
     reent->_errno = ENOMEM;
 
-    p = (void *)-1;
+    return ((void *)-1);
   }
-  else {
-    /* All good, extend heap space */
-    heap += incr;
+  #endif
 
-    p = (void *)heap_prev;
-  }
+  heap += incr;
 
-  return (p);
+  return ((void *)heap_prev);
 }