cmd/compile/internal/amd64: simplify code generation for signed division

The same switch statement handles code generation for signed division of
words, double words and quad words. Rather than using multiple switch
statements to select the appropriate instructions, determine all of the
correctly sized operands up front, then use them as needed.

Updates #59089

Change-Id: I2b7567c8e0ecb9904c37607332538c95b0521dca
Reviewed-on: https://go-review.googlesource.com/c/go/+/482657
Run-TryBot: Joel Sing <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: 4a6f656c

src/cmd/compile/internal/amd64/ssa.go

@@ -338,18 +338,20 @@ func ssaGenValue(s *ssagen.State, v *ssa.Value) {
 		r := v.Args[1].Reg()
 		var j1 *obj.Prog
 
+		var opCMP, opNEG, opSXD obj.As
+		switch v.Op {
+		case ssa.OpAMD64DIVQ:
+			opCMP, opNEG, opSXD = x86.ACMPQ, x86.ANEGQ, x86.ACQO
+		case ssa.OpAMD64DIVL:
+			opCMP, opNEG, opSXD = x86.ACMPL, x86.ANEGL, x86.ACDQ
+		case ssa.OpAMD64DIVW:
+			opCMP, opNEG, opSXD = x86.ACMPW, x86.ANEGW, x86.ACWD
+		}
+
 		// CPU faults upon signed overflow, which occurs when the most
 		// negative int is divided by -1. Handle divide by -1 as a special case.
 		if ssa.DivisionNeedsFixUp(v) {
-			var c *obj.Prog
-			switch v.Op {
-			case ssa.OpAMD64DIVQ:
-				c = s.Prog(x86.ACMPQ)
-			case ssa.OpAMD64DIVL:
-				c = s.Prog(x86.ACMPL)
-			case ssa.OpAMD64DIVW:
-				c = s.Prog(x86.ACMPW)
-			}
+			c := s.Prog(opCMP)
 			c.From.Type = obj.TYPE_REG
 			c.From.Reg = r
 			c.To.Type = obj.TYPE_CONST
@@ -358,17 +360,8 @@ func ssaGenValue(s *ssagen.State, v *ssa.Value) {
 			j1.To.Type = obj.TYPE_BRANCH
 		}
 
-		// Sign extend dividend.
-		switch v.Op {
-		case ssa.OpAMD64DIVQ:
-			s.Prog(x86.ACQO)
-		case ssa.OpAMD64DIVL:
-			s.Prog(x86.ACDQ)
-		case ssa.OpAMD64DIVW:
-			s.Prog(x86.ACWD)
-		}
-
-		// Issue divide.
+		// Sign extend dividend and perform division.
+		s.Prog(opSXD)
 		p := s.Prog(v.Op.Asm())
 		p.From.Type = obj.TYPE_REG
 		p.From.Reg = r
@@ -380,15 +373,7 @@ func ssaGenValue(s *ssagen.State, v *ssa.Value) {
 
 			// Issue -1 fixup code.
 			// n / -1 = -n
-			var n1 *obj.Prog
-			switch v.Op {
-			case ssa.OpAMD64DIVQ:
-				n1 = s.Prog(x86.ANEGQ)
-			case ssa.OpAMD64DIVL:
-				n1 = s.Prog(x86.ANEGL)
-			case ssa.OpAMD64DIVW:
-				n1 = s.Prog(x86.ANEGW)
-			}
+			n1 := s.Prog(opNEG)
 			n1.To.Type = obj.TYPE_REG
 			n1.To.Reg = x86.REG_AX