cmd/compile: improve register overwrite decision for resultInArg0 ops
When we're compiling a resultInArg0 op, we need to clobber the
register containing the input value. So we first make a register copy
of the input value. We can then clobber either of the two registers
the value is in and still have the original input value in a register
for future uses.
Before this CL, we always clobbered the original, not the copy.
But that's not always the right decision - if the original is already
in a specific register that it needs to be in later (typically, a
return value register), clobber the copy instead.
This optimization can remove a mov instruction. It saves 1376 bytes
of instructions in cmd/go.
Redo of CL 460656, reverted at CL 463475, with a fix for s390x.
The new code just ensures that the copied value is in a register
which is a valid input register for the instruction.
Change-Id: Id570b8a60a6d2da9090de80a90b6bb0266e9e38a
Reviewed-on: https://go-review.googlesource.com/c/go/+/463221
Auto-Submit: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Keith Randall <khr@google.com> Reviewed-by: Cherry Mui <cherryyz@google.com>