Re: [PATCH v2] combine: Fix ICE in try_combine on pr112494.c [PR112560]

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

On Thu, Mar 07, 2024 at 10:04:32PM +0100, Uros Bizjak wrote:

[snip]

> The part we want to fix deals with the *user* of the CC register. It
> is not true that this is always COMPARISON_P, so EQ, NE, GE, LT, ...
> in the form of
> 
> (LT:CCGC (reg:CCGC 17 flags) (const_int 0))
> 
> but can be something else, such as the above noted
> 
>  (unspec:DI [
>          (reg:CC 17 flags)
>      ] UNSPEC_PUSHFL)

But that is invalid RTL?  The only valid use of a CC is written as
cc-compared-to-0.  It means "the result of something compared to 0,
stored in that cc reg".

(And you can copy a CC reg around, but that is not a use ;-) )

> The source code that deals with the *user* of the CC register assumes
> the former form, so it blindly tries to update the mode of the CC
> register inside LT comparison RTX

Would you like it better if there was an assert for this?  There are
very many RTL requirements that aren't chacked for currently :-/

> (some other nearby source code even
> checks for (const_int 0) RTX). Obviously, this is not the case with
> the former form, where the update tries to:
> 
> SUBST (XEXP (*cc_use_loc, 0), ...)
> 
> on unspec, which has no XEXP (..., 0).
> 
> And *this* is what triggers RTX checking assert.

The unspec should have the CC compared with 0 as argument.


Segher