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import from Julia: "fix a possible flisp memory bug", by Jeff Bezanson

This is 1a81a4770cba7b078a28a52dfd5bb630977561c9 in Julia:

"in very rare circumstances this could cause a closure to spill over the heap".

--- a/flisp.c

+++ b/flisp.c

@@ -287,7 +287,7 @@

 	assert(n > 0);

 	n = LLT_ALIGN(n, 2);   // only allocate multiples of 2 words

-	if(__unlikely((value_t*)FL(curheap) > ((value_t*)FL(lim))+2-n)){

+	if(__unlikely((value_t*)FL(curheap) > (value_t*)FL(lim)+2-n)){

 		gc(0);

 		while((value_t*)FL(curheap) > ((value_t*)FL(lim))+2-n)

 			gc(1);

@@ -521,8 +521,12 @@

 		}

 		FL(grew) = !FL(grew);

 	}

-	if(FL(curheap) > FL(lim))  // all data was live

+	if(__unlikely((value_t*)FL(curheap) > (value_t*)FL(lim)-2)){

+		// all data was live; gc again and grow heap.

+		// but also always leave at least 4 words available, so a closure

+		// can be allocated without an extra check.

 		gc(0);

+	}

 }

 static void

@@ -1191,7 +1195,7 @@

 			}else{

 				PUSH(FL(stack)[bp]); // env has already been captured; share

 			}

-			if(FL(curheap) > FL(lim)-2)

+			if(__unlikely((value_t*)FL(curheap) > (value_t*)FL(lim)-2))

 				gc(0);

 			pv = (value_t*)FL(curheap);

 			FL(curheap) += 4*sizeof(value_t);