cmd/compile: regalloc: handle desired registers of 2-output insns

Particularly with 2-word load instructions, this becomes important.
Classic example is:

    func f(p *string) string {
        return *p
    }

We want the two loads to put the return values directly into
the two ABI return registers.

At this point in the stack, cmd/go is 1.1% smaller.

Change-Id: I51fd1710238e81d15aab2bfb816d73c8e7c207b1
Reviewed-on: https://go-review.googlesource.com/c/go/+/631137
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/cmd/compile/internal/ssa/regalloc.go

@@ -930,6 +930,7 @@ func (s *regAllocState) regalloc(f *Func) {
 
 	// Data structure used for computing desired registers.
 	var desired desiredState
+	desiredSecondReg := map[ID][4]register{} // desired register allocation for 2nd part of a tuple
 
 	// Desired registers for inputs & outputs for each instruction in the block.
 	type dentry struct {
@@ -949,6 +950,7 @@ func (s *regAllocState) regalloc(f *Func) {
 		s.curBlock = b
 		s.startRegsMask = 0
 		s.usedSinceBlockStart = 0
+		clear(desiredSecondReg)
 
 		// Initialize regValLiveSet and uses fields for this block.
 		// Walk backwards through the block doing liveness analysis.
@@ -1346,6 +1348,11 @@ func (s *regAllocState) regalloc(f *Func) {
 				}
 				dinfo[i].in[j] = desired.get(a.ID)
 			}
+			if v.Op == OpSelect1 && prefs[0] != noRegister {
+				// Save desired registers of select1 for
+				// use by the tuple generating instruction.
+				desiredSecondReg[v.Args[0].ID] = prefs
+			}
 		}
 
 		// Process all the non-phi values.
@@ -1748,6 +1755,17 @@ func (s *regAllocState) regalloc(f *Func) {
 							}
 						}
 					}
+					if out.idx == 1 {
+						if prefs, ok := desiredSecondReg[v.ID]; ok {
+							for _, r := range prefs {
+								if r != noRegister && (mask&^s.used)>>r&1 != 0 {
+									// Desired register is allowed and unused.
+									mask = regMask(1) << r
+									break
+								}
+							}
+						}
+					}
 					// Avoid registers we're saving for other values.
 					if mask&^desired.avoid&^s.nospill&^s.used != 0 {
 						mask &^= desired.avoid
@@ -2874,7 +2892,8 @@ type desiredStateEntry struct {
 	// Registers it would like to be in, in priority order.
 	// Unused slots are filled with noRegister.
 	// For opcodes that return tuples, we track desired registers only
-	// for the first element of the tuple.
+	// for the first element of the tuple (see desiredSecondReg for
+	// tracking the desired register for second part of a tuple).
 	regs [4]register
 }
 

test/codegen/memcombine.go

@@ -1004,6 +1004,16 @@ func dwloadArg(a [2]int64) int64 {
 	return a[0] + a[1]
 }
 
+func dwloadResult1(p *string) string {
+	// arm64:"LDP\t\\(R0\\), \\(R0, R1\\)"
+	return *p
+}
+
+func dwloadResult2(p *[2]int64) (int64, int64) {
+	// arm64:"LDP\t\\(R0\\), \\(R1, R0\\)"
+	return p[1], p[0]
+}
+
 // ---------------------------------- //
 //    Arm64 double-register stores    //
 // ---------------------------------- //