[Mesa-dev] [PATCH 5/7] i965/fs: Don't dead code eliminate CMP(N).

[Eric Anholt]

Matt Turner <mattst88 at gmail.com> writes:

> An future commit will enable CSE for CMP instructions. It operates on
> the following code
>
>       cmp.ge.f0(8)  null     g45<8,8,1>F  0F
> (+f0) sel(8)        g50<1>F  g40<8,8,1>F  g10<8,8,1>F
>       cmp.ge.f0(8)  null     g45<8,8,1>F  0F
> (+f0) sel(8)        g51<1>F  g41<8,8,1>F  g11<8,8,1>F
>       cmp.ge.f0(8)  null     g45<8,8,1>F  0F
> (+f0) sel(8)        g52<1>F  g42<8,8,1>F  g12<8,8,1>F
>       cmp.ge.f0(8)  null     g45<8,8,1>F  0F
> (+f0) sel(8)        g53<1>F  g43<8,8,1>F  g13<8,8,1>F
>
> by adding a new cmp.ge.f0(8) instruction writing into a GRF destination
> and replacing the existing compares with MOVs from that destination,
> leading to:
>
>       cmp.ge.f0(8)  g30<1>F  g45<8,8,1>F  0F
>       mov(8)        null     g30<1>F
> (+f0) sel(8)        g50<1>F  g40<8,8,1>F  g10<8,8,1>F
>       mov(8)        null     g30<1>F
> (+f0) sel(8)        g51<1>F  g41<8,8,1>F  g11<8,8,1>F
>       mov(8)        null     g30<1>F
> (+f0) sel(8)        g52<1>F  g42<8,8,1>F  g12<8,8,1>F
>       mov(8)        null     g30<1>F
> (+f0) sel(8)        g53<1>F  g43<8,8,1>F  g13<8,8,1>F
>
> The next patch will enable dead code elimination of MOVs with a null
> destination, which in turn would cause the pass to recognize that the
> new compare's g30 destination is never read and delete the instruction.
> This would leave the flag register in an unknown state and break the
> program.

I think we should just not generate the dead MOVs, instead.  It would
just be a matter of checking for dst.file == GRF when generating the
temporary, right?
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