From: Vincent MAILHOL
Sent: 28 January 2024 06:27
On Sun. 28 Jan. 2024 at 14:39, Finn Thain <fthain@xxxxxxxxxxxxxx> wrote:
On Sun, 28 Jan 2024, Vincent Mailhol wrote:
The compiler is not able to do constant folding on "asm volatile" code.
Evaluate whether or not the function argument is a constant expression
and if this is the case, return an equivalent builtin expression.
...
If the builtin has the desired behaviour, why do we reimplement it in asm?
Shouldn't we abandon one or the other to avoid having to prove (and
maintain) their equivalence?
The asm is meant to produce better results when the argument is not a
constant expression. Below commit is a good illustration of why we
want both the asm and the built:
https://git.kernel.org/torvalds/c/146034fed6ee
I say "is meant", because I did not assert whether this is still true.
Note that there are some cases in which the asm is not better anymore,
for example, see this thread:
https://lore.kernel.org/lkml/20221106095106.849154-2-mailhol.vincent@xxxxxxxxxx/
but I did not receive more answers, so I stopped trying to investigate
the subject.
If you want, you can check the produced assembly of both the asm and
the builtin for both clang and gcc, and if the builtin is always
either better or equivalent, then the asm can be removed. That said, I
am not spending more effort there after being ghosted once (c.f. above
thread).
I don't see any example there of why the __builtin_xxx() versions
shouldn't be used all the time.
(The x86-64 asm blocks contain unrelated call instructions and objdump
wasn't passed -d to show what they were.
One even has the 'return thunk pessimisation showing.)
I actually suspect the asm versions predate the builtins.
Does (or can) the outer common header use the __builtin functions
if no asm version exists?
David
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