regrename: treat writes as reads for fused instruction pairs

Consider the following (RISC-V) instruction pair:

mul s6,a1,a2
add s6,a4,s6

Without this patch, while handling the second instruction, (a) the
existing chain for s6 will first be closed (upon the terminate_write
action for the input operand), then (b) a new one will be opened (upon
the mark_write action for the output operand).  This will likely lead to
the two output operands being different physical registers, breaking the
single-output property required for some macro-op fusion pairs.

This patch, using the single_output_fused_pair_p () predicate introduced
earlier, changes the regrename behavior for such pairs to append the
input and the output operands to the existing chain (as if both actions
were mark_read), instead of breaking the current renaming chain and
starting a new one.  This ensures that the output operands of both fused
instructions are kept in the same hard register, and that the
single-output property of the insn pair is preserved.

gcc/ChangeLog:

	* regrename.cc (scan_rtx_reg): Handle fused insn pairs.

0 files changed, 0 insertions, 0 deletions