Matt Turner <matts...@gmail.com> writes:
> Since compare instructions write the flag register, they should not be
> considered dead even if their destination is never read. Instead of
> removing them if found to be dead, set their destination to null to free
> a register.
>
> ---
> src/mesa/drivers/dri/i965/brw_fs.cpp | 6 ++++--
> 1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/src/mesa/drivers/dri/i965/brw_fs.cpp
> b/src/mesa/drivers/dri/i965/brw_fs.cpp
> index 7e7ced0..b985251 100644
> --- a/src/mesa/drivers/dri/i965/brw_fs.cpp
> +++ b/src/mesa/drivers/dri/i965/brw_fs.cpp
> @@ -1903,10 +1903,12 @@ fs_visitor::dead_code_eliminate()
>
> if (dead) {
> /* Don't dead code eliminate instructions that write to the
> - * accumulator as a side-effect. Instead just set the destination
> - * to the null register to free it.
> + * accumulator or flag register as a side effect. Instead just
> set
> + * the destination to the null register to free it.
> */
> switch (inst->opcode) {
> + case BRW_OPCODE_CMP:
> + case BRW_OPCODE_CMPN:
> case BRW_OPCODE_ADDC:
> case BRW_OPCODE_SUBB:
> case BRW_OPCODE_MACH:I don't think this one is needed in the series any more after the rework to not generate new CMP.whatever.f0 to a non-null destination in CSE, but I think it should still land anyway. I'd prefer the test for "writes to the flag register" to be inst->writes_flag() rather than just a couple of opcodes that always write the flag, though.
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