Re: [PATCH 0/6] tcg: Always implement neg and movcond

[Richard Henderson <richard.henderson@linaro.org>]

On 10/25/23 21:13, Richard Henderson wrote:
> Having opcodes always present means that we can remove some tests.
> NOT and MOVCOND were *almost* always present anyway.
> 
> A close candidate is NOT.  The hiccup is s390x: except for the most
> recent hardware revision, there is no single insn which can implement
> the operation.
> 
> I experimented with replacements:
> 
>      i32: xilf r,-1             (6 bytes, requires R=X)
>           lcr r,x; ahi r,-1     (6 bytes)
>      i64: lcgr r,x; aghi r,-1   (8 bytes)
> 
> But both don't compare well with the current
> 
>           lghi tmp,-1           (4 bytes, shared)
>           xgrk r,x,tmp          (4 bytes)
> 
> With the constant -1 managed by register allocation, it gets loaded
> once and then reused between operations.
> 
> An alternative might be to assume the presence of all opcodes during
> initial expansion and lower them later, after optimization.  I'm close
> to doing just that for the more complex opcodes like deposit anyway,
> because the expansion is too complex for the optimizer to do anything
> sensible after constant propagation.

Oh, for the record,

Based-on: 20231025072707.833943-1-richard.henderson@linaro.org
("tcg: Introduce TCG_COND_TST{EQ,NE}")


r~