cmd/internal/obj: ARM, use immediates instead of constant pool entries
When a constant doesn't fit in a single instruction, use two
paired instructions instead of the constant pool. For example
ADD $0xaa00bb, R0, R1
Used to rewrite to:
MOV ?(IP), R11
ADD R11, R0, R1
Instead, do:
ADD $0xaa0000, R0, R1
ADD $0xbb, R1, R1
Same number of instructions.
Good:
4 less bytes (no constant pool entry)
One less load.
Bad:
Critical path is one instruction longer.
It's probably worth it to avoid the loads, they are expensive.
Dave Cheney got us some performance numbers: https://perf.golang.org/search?q=upload:
20170426.1
TL;DR mean 1.37% improvement.
Change-Id: Ib206836161fdc94a3962db6f9caa635c87d57cf1
Reviewed-on: https://go-review.googlesource.com/41612
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>