Re: GCC plays "Shell Game", but looses track of the shell covering the nought

["Stefan Kanthak" <stefan.kanthak@nexgo.de>]

"Dave Blanchard" <dave@killthe.net> wrote:

> Hi Stefan, thanks for sharing this information.
> I was wondering if the code generators in earlier GCC
> versions were any better?

Just open one of the URLs I included, select another GCC version
and see the resulting code.

> Is this a problem in GCC 12+ only?

NO! GCC's code generator REALLY sucks, especially when "double word"
operations are involved.

GCC 6.* generates the following "gem":

.L5: # eax is already 0 here!
        xor     eax, eax
        movzx   eax, al
        pop     ebx
        pop     esi
        ret

GCC 7.* is even worse, it clobbers FOUR registers.

GCC 8.* and 9.* uses/clobbers just one additional register.

GCC 10.* generates the same code as GCC 13.1 with "only" 13 superfluous
instructions (from a total of 26).

GCC 11.* shows the same behaviour with 19 superfluous instructions as 12.*

Also note the difference to yesterdays demo.c: "thanks" to the added
| (argument != 0)
GCC does NOT generate SSE2 instructions any more.

I don't know yet whether this change is a quirk or WTF,

Stefan

> Dave
> 
> 
> On Sat, 27 May 2023 18:23:12 +0200
> "Stefan Kanthak" <stefan.kanthak@nexgo.de> wrote:
> 
>> [...]
>> It's a REAL shame how bad GCC's code generator is!
>> 
>> Stefan