From: David Malcolm
> Sent: 09 April 2021 14:49
...
> With the caveat that my knowledge of GCC's middle-end is mostly about
> implementing warnings, rather than optimization, I did some
> experimentation, with gcc trunk on x86_64 FWIW.
>
> Given:
>
> int __attribute__((pure)) foo(void);
>
> int t(void)
> {
> int a;
= 0;
> if (foo())
> a++;
> if (foo())
> a++;
> if (foo())
> a++;
> return a;
> }
>
> At -O1 and above this is optimized to a single call to foo, returning 0
> or 3 accordingly.
Interesting horrid idea.
The code generated for the above should be:
call foo
jz label
So objtool could find the relocation entries for 'foo'
and use that information to replace the call instruction
(and maybe the jz) with a suitable alternate instruction
sequence.
Although that might end up with a game of 'whack-a-mole'
on the perverse instruction sequences the compiler
generates.
David
-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT,
UK
Registration No: 1397386 (Wales)
