* WASI: preserve the initial value of the stack pointer on startup, so that we

  can later use the area after that, until the end of memory as an initial heap

rtl/wasi/si_dll.pp

@@ -32,7 +32,7 @@ procedure PASCALMAIN; external 'PASCALMAIN';
 {$if defined(FPC_WASM_BRANCHFUL_EXCEPTIONS) or defined(FPC_WASM_NATIVE_EXCEPTIONS)}
 Procedure DoUnHandledException; external name 'FPC_DOUNHANDLEDEXCEPTION';
 
-procedure _initialize;
+procedure _initialize_pascal;
 begin
   try
     PASCALMAIN;
@@ -41,12 +41,28 @@ begin
   end;
 end;
 {$else}
-procedure _initialize;
+procedure _initialize_pascal;
 begin
   PASCALMAIN;
 end;
 {$endif}
 
+procedure SetInitialHeapBlockStart(p: Pointer); external name 'FPC_WASM_SETINITIALHEAPBLOCKSTART';
+
+{ TODO: remove this, when calling SetInitialHeapBlockStart works directly from within inline asm }
+procedure SetInitialHeapBlockStart2(p: Pointer);
+begin
+  SetInitialHeapBlockStart(p);
+end;
+
+procedure _initialize; assembler; nostackframe;
+asm
+  global.get $__stack_pointer
+  call $SetInitialHeapBlockStart2
+
+  call $_initialize_pascal
+end;
+
 exports
   _initialize,
   _initialize name '_initialize_promising' promising;

rtl/wasi/si_prc.pp

@@ -32,7 +32,7 @@ procedure PASCALMAIN; external 'PASCALMAIN';
 {$if defined(FPC_WASM_BRANCHFUL_EXCEPTIONS) or defined(FPC_WASM_NATIVE_EXCEPTIONS)}
 Procedure DoUnHandledException; external name 'FPC_DOUNHANDLEDEXCEPTION';
 
-procedure _start;
+procedure _start_pascal;
 begin
   try
     PASCALMAIN;
@@ -41,12 +41,28 @@ begin
   end;
 end;
 {$else}
-procedure _start;
+procedure _start_pascal;
 begin
   PASCALMAIN;
 end;
 {$endif}
 
+procedure SetInitialHeapBlockStart(p: Pointer); external name 'FPC_WASM_SETINITIALHEAPBLOCKSTART';
+
+{ TODO: remove this, when calling SetInitialHeapBlockStart works directly from within inline asm }
+procedure SetInitialHeapBlockStart2(p: Pointer);
+begin
+  SetInitialHeapBlockStart(p);
+end;
+
+procedure _start; assembler; nostackframe;
+asm
+  global.get $__stack_pointer
+  call $SetInitialHeapBlockStart2
+
+  call $_start_pascal
+end;
+
 exports
   _start,
   _start name '_start_promising' promising;

rtl/wasi/sysheap.inc

@@ -20,6 +20,14 @@
                               Heap Management
 *****************************************************************************}
 
+var
+  InitialHeapBlockStart: Pointer;
+
+procedure SetInitialHeapBlockStart(p: Pointer);[Public, Alias : 'FPC_WASM_SETINITIALHEAPBLOCKSTART'];
+begin
+  InitialHeapBlockStart:=p;
+end;
+
 function SysOSAlloc(size: ptruint): pointer;
 const
   page_size = 65536;