From 4a0e84a1be52d9a57574de0421d9aa38522a0b71 Mon Sep 17 00:00:00 2001 From: Keith Randall Date: Thu, 5 Jan 2023 13:02:44 -0800 Subject: [PATCH] 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 Run-TryBot: Keith Randall TryBot-Result: Gopher Robot Reviewed-by: Keith Randall Reviewed-by: Cherry Mui --- src/cmd/compile/internal/ssa/regalloc.go | 16 ++++++++++++++++ 1 file changed, 16 insertions(+) diff --git a/src/cmd/compile/internal/ssa/regalloc.go b/src/cmd/compile/internal/ssa/regalloc.go index 294c522a90..80f6434e76 100644 --- a/src/cmd/compile/internal/ssa/regalloc.go +++ b/src/cmd/compile/internal/ssa/regalloc.go @@ -1544,6 +1544,7 @@ func (s *regAllocState) regalloc(f *Func) { } } } + // Avoid future fixed uses if we can. if m&^desired.avoid != 0 { m &^= desired.avoid @@ -1551,6 +1552,21 @@ func (s *regAllocState) regalloc(f *Func) { // Save input 0 to a new register so we can clobber it. c := s.allocValToReg(v.Args[0], m, true, v.Pos) s.copies[c] = false + + // Normally we use the register of the old copy of input 0 as the target. + // However, if input 0 is already in its desired register then we use + // the register of the new copy instead. + if regspec.outputs[0].regs>>s.f.getHome(c.ID).(*Register).num&1 != 0 { + if rp, ok := s.f.getHome(args[0].ID).(*Register); ok { + r := register(rp.num) + for _, r2 := range dinfo[idx].in[0] { + if r == r2 { + args[0] = c + break + } + } + } + } } ok: -- 2.48.1