Reload Issue -- I can't believe we haven't hit this before

[Eric Botcazou]

> So the combination of the TCB merge plus the pending jump threading
> changes apparently has ticked a reload bug which manifests itself with
> the stage1 compiler mis-compiling the stage2 compiler.
>
> [...]
>
> Which faults because the memory location is actually  read-only memory.

PR rtl-optimization/15248.

> What's not clear to me is how best to fix this.
>
> We could try to delete all assignments to pseudos which are equivalent
> to MEMs.
>
> We could avoid recording equivalences when the pseudo is set more than
> once.
>
> Other possibilities?

For 3.3 and 3.4, this was "fixed" by not recording memory equivalences that 
have the infamous RTX_UNCHANGING_P flag set.

-- 
Eric Botcazou