[fpc-pascal] issue when enabling -O2

[Karoly Balogh (Charlie/SGR)]

Hi,

On Wed, 10 Jan 2018, Matias Vara wrote:

> I am getting an exception when I enable the -O2 optimization. More
> precisaily, the line that stars with write_portd.... is corrupting the
> data section. This is the pascal code: 
>
> function PciReadDword(const bus, device, func, regnum: UInt32): UInt32;
> var
>   Send: DWORD;
> begin
>   Send := $80000000 or (bus shl 16) or (device shl 11) or (func shl 8) or (regnum shl 2);
>   write_portd(@Send, PCI_CONF_PORT_INDEX);
>   read_portd(@Send, PCI_CONF_PORT_DATA);
>   Result := Send;
> end;  
>
> which generates (without -02):
>
> .section .text.n_arch_$$_pcireaddword$longword$longword$longword$longword$$longword,"x"
> .balign 16,0x90
> .globl ARCH_$$_PCIREADDWORD$LONGWORD$LONGWORD$LONGWORD$LONGWORD$$LONGWORD
> ARCH_$$_PCIREADDWORD$LONGWORD$LONGWORD$LONGWORD$LONGWORD$$LONGWORD:
>
> (***shipp***)
>
> and with -O2:
>
> .section .text.n_arch_$$_pciwriteword$word$word$word$word$word,"x"
> .balign 16,0x90
> .globl ARCH_$$_PCIWRITEWORD$WORD$WORD$WORD$WORD$WORD
> ARCH_$$_PCIWRITEWORD$WORD$WORD$WORD$WORD$WORD:
>
> The first thing that I realize was the the optimized version is not
> generating the correct source when is exiting since it should return
> "Send", but am I right? The assembler code of write_portd remains the
> same, Am I missing something? 

The -O2 version of the function you sent is from a different one, it's
from a Write function, not a Read... So no wonder it doesn't return
anything... :)

BTW, -O2 uses register variables, while -O- doesn't. If your assembler
trashes one of them without preserving one, it can lead to crashes...
But there could be many other reasons.

Without seeing the actual assembler functions it's hard to tell.

Charlie