[PATCH v3] Implement new RTL optimizations pass: fold-mem-offsets.

[Jeff Law]

On 8/7/23 08:44, Manolis Tsamis wrote:
> I have sent out a new v4 of this
> (https://gcc.gnu.org/pipermail/gcc-patches/2023-August/626502.html).
> 
> In the new version I both restore the INSN_CODE as mentioned here and
> I also call recog when I commit the offset change (because there may
> be a change from no offset to some offsets).
> I have also removed the driver function as per Vineet's suggestion.
> 
> Last thing I have fixed a nasty bug with REG_EQUIV's that resulted in
> failing bootstrap on AArch64.
> 
> The offending sequence looked like this (in 'simplified RTX'):
> 
> (set (reg/f:DI 3 x3)
>          (plus:DI (reg/f:DI 2 x2)
>              (const_int 8 [0x8])))
> (set (reg/f:DI 19 x19)
>          (plus:DI (reg/f:DI 3 x3)
>              (reg:DI 19 x19)))
> ...
> (set (reg:TI 2 x2)
>          (mem:TI (reg/f:DI 0 x0)))
>       (expr_list:REG_DEAD (reg/f:DI 0 x0)
>          (expr_list:REG_EQUIV (mem:TI (reg/f:DI 19 x19))
>              (nil)))
> (set (mem:TI (reg/f:DI 19 x19))
>          (reg:TI 2 x2))
>       (expr_list:REG_DEAD (reg/f:DI 19 x19)
>          (expr_list:REG_DEAD (reg:TI 2 x2)
>              (nil)))
> 
> Were the first instruction (x3 = x2 + 8) was folded in the address
> calculation of the last one, resulting in (mem:TI (plus:DI (reg/f:DI
> 19 x19) (const_int 8 [0x8])), but then the previous REG_EQUIV was
> incorrect due to the modified runtime value of x19.
> For now I opted to treat REG_EQ/EQUIV notes as uses that we cannot
> handle, so if any of them are involved we don't fold offsets. Although
> it would may be possible to improve this in the future, I think it's
> fine for now and the reduction of folded insns is a small %.
> I have tested v4 with a number of benchmarks and large projects on
> x64, aarch64 and riscv64 without observing any issues. The x86
> testsuite issue still exists as I don't have a satisfactory solution
> to it yet.
> 
> Any feedback for the changes is appreciated!
Thanks for the explanation.  You could just remove the REG_EQUIV note. 
It's not used much after allocation & reloading.  Or just ignoring those 
insns as you've done seems reasonable as well.  I doubt one approach is 
clear better than the other.

jeff