[AMDGPU] Eliminate redundant s_round_mode and s_denorm_mode writes (#216305)
Add a block local peephole that drops writes to the FP round mode and
the FP denorm mode which cannot be observed. A write is removed when a
later write replaces the field before any instruction reads MODE, or
when the value it writes is already live in MODE.
s_round_mode and s_denorm_mode each assign one field of MODE and
preserve the rest of the register, so the two fields are tracked
independently: a write to one field is transparent to a pending write to
the other.
The analysis is intra-block only: the mode on entry to a block is
treated as unknown, and a write still live at the end of a block is kept
for its successors.
llvm-project
6de78173 - [AMDGPU] Eliminate redundant s_round_mode and s_denorm_mode writes (#216305)
