[RFC][PATCH] Change default to -fcommon

[Wilco Dijkstra]

Michael Matz wrote:

> bss _sections_ != bss-like segments in the executable.  Targets might not 
> have a bss section that could be named in the asm file, or no way to 
> switch to it without disrupting surrounding code, but they might have 
> common symbols, which ultimately might or might not be collected in some 
> bss-like segment.  In that case you want to use them instead of symbols in 
> .data.

OK, thanks for the explanation. For large symbols it obviously makes sense
to keep the executable size reasonable.

Is this really Ada specific and related to -fcommon? We could do this optimization
without checking flag_no_common.

> What's your rationale for changing this?  In your initial mail you said:
>
> "On many targets this means global variable accesses having an unnecessary 
> codesize and performance penalty in C code (the same source generates 
> better code when built as C++)."
>
> I have a hard time imaging that, so can you give details?  FWIW I've 
> personally always considered using common symbols nicer.

Basically -fcommon disables the anchor optimization on RISC targets,
here is a simple example on AArch64:

int a, b, c;
int f (void) { return a + b + c; }

With -fcommon:

f:
	adrp	x1, a
	adrp	x0, b
	adrp	x2, c
	ldr	w1, [x1, #:lo12:a]
	ldr	w0, [x0, #:lo12:b]
	ldr	w2, [x2, #:lo12:c]
	add	w0, w1, w0
	add	w0, w0, w2
	ret

With -fno-common:

f:
	adrp	x0, .LANCHOR0
	add	x2, x0, :lo12:.LANCHOR0
	ldr	w1, [x0, #:lo12:.LANCHOR0]
	ldp	w0, w2, [x2, 4]
	add	w0, w1, w0
	add	w0, w0, w2
	ret

The anchor pointer is set up once and cached across the function, making accesses
to multiple globals cheap and enabling other optimizations. On various targets
(eg. PPC, Arm) creating the address of a global takes 2 instructions so the savings
are larger.

Wilco