Re: ideas on PR/109279

[Jeff Law <jeffreyalaw@gmail.com>]

On 3/31/23 03:11, Vineet Gupta wrote:
> Hi Jeff, Kito,
> 
> I need some ideas to proceed with PR/109279: pertaining to longer term 
> direction and short term fix.
> 
> First the executive summary:
> 
> long long f(void)
> {
>    return 0x0101010101010101ull;
> }
> 
> Up until gcc 12 this used to generate const pool type access.
> 
>      lui    a5,%hi(.LANCHOR0)
>      ld    a0,%lo(.LANCHOR0)(a5)
>      ret
> .LC0:
>      .dword    0x101010101010101
> 
> After commit 2e886eef7f2b ("RISC-V: Produce better code with complex 
> constants [PR95632] [PR106602] ") it gets synthesized to following
> 
> li    a0,0x01010000
>      addi    a0,0x0101
>      slli    a0,a0,16
>      addi    a0,a0,0x0101
>      slli    a0,a0,16
>      addi    a0,a0,0x0101
>      ret
> 
> Granted const pool could or not be preferred by  specific uarch, will 
> the long term approach be to have a cost model for the const pool vs. 
> synthesizing.
> 
> The second aspect is to improve the horror above. Per chat on IRC, 
> pinskia suggested we relax the in_splitter constraint in 
> riscv_move_integer, as the combine issue holding it back is now fixed - 
> after commit 61bee6aed26eb30.
> 
> That beings it down to some what reasonable
> 
>      li        a5,0x01010000
>      addi   a5,a5,0x0101
>      mv     a0,a5
>      slli      a5,a5,32
>      add    a0,a5,a0
>      ret
> 
> I can spin a minimal patch, will that be acceptable for gcc 13.1 if it 
> is testsuite clean 
It would seem to be a gcc-14 thing to me.

It seems like we probably should adjust the basic constant synthesis 
code to handle this class of cases so that the initial RTL is good 
rather than waiting on combine to fix it up.  It looks like we need the 
destination register as well as a temporary and a 5 instruction sequence.

I'm aware of uarch plans that would handle this kind of sequence 
entirely in the front-end and pass off a single uop to the execution 
units.  We'd planned to dig into constant synthesis in support of that 
effort.  So I'm happy to help shepherd this improvement once gcc-14 
development opens.

jeff