GCC is being push-phobic (?)

[Shaun Spiller]

I've noticed in the disassemblies of code that GCC generates (via the
-save-temps option) that it's trying to avoid pushing arguments onto
the stack when calling functions. Instead, it makes some space on the
stack in the calling function, and movs arguments directly on to it
(even with -Os).

Then, for a different reason, I added -mno-stack-arg-probe to the
command line arguments, and noticed that the number of lines of *.s
output generated in my project had gone up by about 3%, while the size
of the resulting binary had gone down by about 7%. When I looked at
the disassemblies again to try to find out why, I find it's suddenly
passing arguments via push.

Example function follows.

Without -mno-stack-arg-probe:
__ZN6screen6scrollEv:
1	push	ebp
2	mov	ebp, esp
3	push	esi
4	push	ebx
5	sub	esp, 16
6	mov	esi, DWORD PTR __ZL5width
7	sal	esi
8	mov	ebx, DWORD PTR __ZL16framebuffer_size
9	sub	ebx, esi
10	mov	DWORD PTR [esp+8], ebx
11	lea	ecx, [esi+753664]
12	mov	DWORD PTR [esp+4], ecx
13	mov	DWORD PTR [esp], 753664
14	call	__Z7memmovePvPKvj
15	mov	DWORD PTR [esp+8], esi
16	movzx	edx, BYTE PTR __ZL9backcolor
17	sal	edx, 4
18	or	dl, BYTE PTR __ZL9forecolor
19	movzx	eax, dl
20	sal	eax, 8
21	mov	DWORD PTR [esp+4], eax
22	add	ebx, 753664
23	mov	DWORD PTR [esp], ebx
24	call	__Z9memset_16Pvij
25	add	esp, 16
26	pop	ebx
27	pop	esi
28	leave
29	ret

With -mno-stack-arg-probe:
__ZN6screen6scrollEv:
1	push	ebp
2	mov	ebp, esp
3	push	esi
4	push	ebx
5	mov	esi, DWORD PTR __ZL5width
6	sal	esi
7	mov	ebx, DWORD PTR __ZL16framebuffer_size
8	sub	ebx, esi
9	push	ecx
10	push	ebx
11	lea	ecx, [esi+753664]
12	push	ecx
13	push	753664
14	call	__Z7memmovePvPKvj
15	add	esp, 12
16	push	esi
17	movzx	edx, BYTE PTR __ZL9backcolor
18	sal	edx, 4
19	or	dl, BYTE PTR __ZL9forecolor
20	movzx	eax, dl
21	sal	eax, 8
22	push	eax
23	add	ebx, 753664
24	push	ebx
25	call	__Z9memset_16Pvij
26	add	esp, 16
27	lea	esp, [ebp-8]
28	pop	ebx
29	pop	esi
30	leave
31	ret

This behavior seems extremely peculiar. Can anyone shed some light?