[fpc-devel] A bug with temp allocation?

[Sergei Gorelkin]

Hello,

I have noticed a strange thing with FPC-generated assembler code.
The Pascal code is very simple:

program test;
{$mode objfpc}{$h+}
type
   TObj = class(TObject)
   public
     destructor Destroy; override;
   end;

destructor TObj.Destroy;
begin
   writeln('TObj.Destroy');
end;

begin
end.

I compiled with 'fpc test.pp -a -AMASM -al -s -O1'.

The generated assembler code follows below. Look at the prologs/epilogs.
When compiled with -O1, there are 16 bytes allocated at the stack, but 
location [ebp-12] is never used.
When compiled with -O2, however, allocation increases to 32 bytes, but 
still only three DWORDs (12 bytes) are used, remaining 20 bytes look 
just wasted.

Is this subject for a bug report?

Regards,
Sergei

--------------------------------------------------
// assembler code compiled with -O1
_CODE		SEGMENT	PARA PUBLIC USE32 'CODE'
	ALIGN 16
	PUBLIC	P$TEST_TOBJ_$__DESTROY
P$TEST_TOBJ_$__DESTROY:
; Temps allocated between ebp-16 and ebp-8
; [25] begin
		push	ebp
		mov	ebp,esp
		sub	esp,16
		mov	dword ptr [ebp-16],ebx
; Var $vmt located at ebp-4
; Var $self located at ebp-8
		mov	dword ptr [ebp-8],eax
		mov	dword ptr [ebp-4],edx
		mov	eax,dword ptr [ebp-4]
		test	eax,eax
		jng	@@j48
		mov	eax,dword ptr [ebp-8]
		mov	edx,dword ptr [ebp-8]
		mov	edx,dword ptr [edx]
		call	dword ptr [edx+72]
@@j48:
; [26] writeln('TObj.Destroy');
		call	fpc_get_output
		mov	ebx,eax
		push	11
		push	1
		mov	ecx,offset _$TEST$_Ld2
		mov	edx,ebx
		mov	eax,0
		call	fpc_write_text_pchar_as_array
		call	FPC_IOCHECK
		mov	eax,ebx
		call	fpc_writeln_end
		call	FPC_IOCHECK
; [27] end;
		mov	eax,dword ptr [ebp-8]
		test	eax,eax
		je	@@j66
		mov	eax,dword ptr [ebp-4]
		test	eax,eax
		je	@@j66
		mov	eax,dword ptr [ebp-8]
		mov	edx,dword ptr [ebp-8]
		mov	edx,dword ptr [edx]
		call	dword ptr [edx+56]
@@j66:
		mov	ebx,dword ptr [ebp-16]
		leave
		ret
_CODE		ENDS

-------------------------------------------------
// assembler code compiled with -O2
_CODE		SEGMENT	PARA PUBLIC USE32 'CODE'
	ALIGN 16
	PUBLIC	P$TEST_TOBJ_$__DESTROY
P$TEST_TOBJ_$__DESTROY:
; Temps allocated between ebp-32 and ebp+0
; [25] begin
		push	ebp
		mov	ebp,esp
		sub	esp,32
; Var $vmt located in register ebx
; Var $self located in register edi
		mov	dword ptr [ebp-32],ebx
		mov	dword ptr [ebp-28],esi
		mov	dword ptr [ebp-24],edi
		mov	edi,eax
		mov	ebx,edx
		test	ebx,ebx
		jng	@@j48
		mov	eax,edi
		mov	edx,dword ptr [edi]
		call	dword ptr [edx+72]
@@j48:
; [26] writeln('TObj.Destroy');
		call	fpc_get_output
		push	11
		push	1
		mov	ecx,offset _$TEST$_Ld2
		mov	esi,eax
		mov	edx,esi
		mov	eax,0
		call	fpc_write_text_pchar_as_array
		call	FPC_IOCHECK
		mov	eax,esi
		call	fpc_writeln_end
		call	FPC_IOCHECK
; [27] end;
		test	edi,edi
		je	@@j66
		test	ebx,ebx
		je	@@j66
		mov	eax,edi
		mov	edx,dword ptr [edi]
		call	dword ptr [edx+56]
@@j66:
		mov	ebx,dword ptr [ebp-32]
		mov	esi,dword ptr [ebp-28]
		mov	edi,dword ptr [ebp-24]
		leave
		ret
_CODE		ENDS