Re: [PATCH v2, rs6000] Fix ICE on expand bcd<bcd_add_sub>_<code>_<mode> [PR100736]

[Segher Boessenkool <segher@kernel.crashing.org>]

Hi!

On Thu, Jun 02, 2022 at 01:30:04PM +0800, HAO CHEN GUI wrote:
> Segher,
>   Does BCD comparison return false when either operand is invalid coding?

It sets all of LT, GT, and EQ to 0 (it normally sets exactly one of them
to 1).  It sets bit 3 (the "SO" bit usually) to 1.

That is what the machine insns do.  What the builtins do is undefined as
far as I know?  If So we can do whatever is most convenient, so, not
handle it specifically at all, just go with what falls out.

> If yes, the result could be 3-way. We can check gt and eq bits for ge.

You can check the LT bit, instead: it is only one branch insn, and also
only one setbc[r] insn (it can be slightly more expensive if you can use
only older insns).

> We still can't use crnot to only check lt bit as there could be invalid
> coding.
>   Also, do you think finite-math-only excludes invalid coding? Seems GCC
> doesn't clear define it.

This is not floating-point code at all, it should not be influenced at
all by finite-math-only!


Segher