On 6/18/23 04:22, Roger Sayle wrote:
An x86 backend improvement that I'm working results in combine attempting
to recognize:
(set (reg:DI 87 [ xD.2846 ])
(ior:DI (subreg:DI (ashift:TI (zero_extend:TI (reg:DI 92))
(const_int 64 [0x40])) 0)
(reg:DI 91)))
where the lowpart SUBREG has difficulty seeing through the (hi<<64)
that the lowpart must be zero. Rather than workaround this in the
backend, the better fix is to teach simplify-rtx that
lowpart((hi<<64)|lo) -> lo and highpart((hi<<64)|lo) -> hi, so that
all backends benefit. Reducing the number of places where the
middle-end generates a SUBREG of something other than REG is a
good thing.
This patch has been tested on x86_64-pc-linux-gnu with make bootstrap
and make -k check, both with and without --target_board=unix{-m32}
with no new failures, except for pr78904-1b.c, for which a backend
solution has just been proposed. Ok for mainline?
2023-06-18 Roger Sayle <ro...@nextmovesoftware.com>
gcc/ChangeLog
* simplify-rtx.cc (simplify_subreg): Optimize lowpart SUBREGs
of ASHIFT to const0_rtx with sufficiently large shift count.
Optimize highpart SUBREGs of ASHIFT as the shift operand when
the shift count is the correct offset. Optimize SUBREGs of
multi-word logic operations if the SUBREGs of both operands
can be simplified.
OK
Jeff
