+ WASI: use an initial heap area, that exists between the end of the stack area,

  and before the end of initially allocated memory. This recovers some memory
  that was previously wasted and delays the first call to memory.grow

rtl/wasi/sysheap.inc

@@ -20,27 +20,41 @@
                               Heap Management
 *****************************************************************************}
 
+const
+  WasmMemoryPageSize=65536;
+
 var
   InitialHeapBlockStart: Pointer;
+  InitialHeapBlockEnd: Pointer;
 
 procedure SetInitialHeapBlockStart(p: Pointer);[Public, Alias : 'FPC_WASM_SETINITIALHEAPBLOCKSTART'];
 begin
   InitialHeapBlockStart:=p;
 end;
 
+procedure InitInitialHeapBlock;
+begin
+  InitialHeapBlockEnd:=Pointer(PtrUInt(fpc_wasm32_memory_size)*WasmMemoryPageSize);
+end;
+
 function SysOSAlloc(size: ptruint): pointer;
 const
-  page_size = 65536;
   err = high(longword);
 var
   res: ptruint;
 begin
-  res:=fpc_wasm32_memory_grow((size + page_size - 1) div page_size);
-  if res<>err then
-    SysOSAlloc:=pointer(res*page_size)
+  if (PtrUInt(InitialHeapBlockEnd)-PtrUInt(InitialHeapBlockStart))>=size then
+    begin
+      SysOSAlloc:=InitialHeapBlockStart;
+      Inc(InitialHeapBlockStart,size);
+    end
   else
     begin
-    SysOSAlloc:=nil;
+      res:=fpc_wasm32_memory_grow((size + WasmMemoryPageSize - 1) div WasmMemoryPageSize);
+      if res<>err then
+        SysOSAlloc:=pointer(res*WasmMemoryPageSize)
+      else
+        SysOSAlloc:=nil;
     end;
 end;
 

rtl/wasi/system.pp

@@ -435,6 +435,7 @@ begin
   IsLibrary := FALSE;
 {$endif def FPC_HAS_FEATURE_DYNLIBS}
   { Setup heap }
+  InitInitialHeapBlock;
   InitHeap;
   SysInitExceptions;
   initunicodestringmanager;