Re: [PATCH] combine: Don't optimize SIGN_EXTEND of MEM on WORD_REGISTER_OPERATIONS targets [PR113010]

[Greg McGary <gkm@rivosinc.com>]

On 1/18/24 9:24 AM, Jeff Law wrote:
>
> On 1/17/24 20:53, Greg McGary wrote:
>>
>> While the code comment is true, perhaps it obscures the primary intent,
>> which is recognition that the pattern (SIGN_EXTEND (mem ...) ) is 
>> destined
>> to expand into a single memory-load instruction and no simplification is
>> possible, so why waste time with further analysis or transformation? 
>> There
>> are plenty of other conditions that also short circuit to "do 
>> nothing" and
>> this seems just as straightforward as those others. Efforts to catch 
>> this
>> further downstream add gratuitous complexity.
> Because the real bug is likely still lurking, waiting for something 
> else to trigger it.
>
> An early exit is fine when we're just trying to avoid unnecessary 
> work, but there's something else going on here we need to understand 
> first.


expand_compound_operation() wants to evaluate the sign- and 
zero-extension of MEM. It begins by calling gen_lowpart(), which returns 
the same result in both cases: a paradoxical subreg of a MEM (PSoM). 
What is the value of the high part of a PSoM? Can that high part be 
evaluated at compile time?

According to existing code, the high part of a PSoM is statically 0. 
However, for a machine where (WORD_REGISTER_OPERATIONS && load_extend_op 
(inner_mode) == SIGN_EXTEND), the high part of a PSoM is  only known at 
runtime as 0s or 1s. That's the downstream bug. The fix for such 
machines is either (A) forbid static evaluation of the high part of a 
PSoM, or (B) forbid transforming (SIGN_EXTEND (MEM ...) ) into a PSoM. 
My patch does B. Perhaps you prefer A? The trouble with A is that in the 
zero-extend case, it is valid to statically evaluate as 0. It is only 
the sign-extend case that isn't known until runtime. By the time we have 
a PSoM, its upstream ancestor as sign- or zero-extend is already lost.

Does that give you the understanding you desire, or are there deeper 
mysteries to probe?

G