liblink, cmd/internal/obj: prefer positive immediates

Rewrite instructions that use negative immediates as the "negative"
instruction variant with a positive operand.

E.g. ADD $-2, R becomes SUB $2, R

Updates golang#6

Author: 4ad

src/cmd/internal/obj/arm64/asm7.go

@@ -818,7 +818,7 @@ func addpool(ctxt *obj.Link, p *obj.Prog, a *obj.Addr) {
 		so we need to know what to do with C_MOVCON.
 
 		The correct fix is to use the "negation" instruction variant,
-		e.g. CMN $0, R instead of CMP $-1, R, or SUB $1, R instead
+		e.g. CMN $1, R instead of CMP $-1, R, or SUB $1, R instead
 		of ADD $-1, R. */
 	case C_MOVCON,
 

src/cmd/internal/obj/arm64/obj7.go

@@ -88,6 +88,17 @@ func settextflag(p *obj.Prog, f int) {
 	p.Reg = uint8(f)
 }
 
+var complements = []int16{
+	AADD:  ASUB,
+	AADDW: ASUBW,
+	ASUB:  AADD,
+	ASUBW: AADDW,
+	ACMP:  ACMN,
+	ACMPW: ACMNW,
+	ACMN:  ACMP,
+	ACMNW: ACMPW,
+}
+
 func progedit(ctxt *obj.Link, p *obj.Prog) {
 	var literal string
 	var s *obj.LSym
@@ -142,6 +153,26 @@ func progedit(ctxt *obj.Link, p *obj.Prog) {
 
 		break
 	}
+
+	// Rewrite negative immediates as positive immediates with
+	// complementary instruction.
+	switch p.As {
+
+	case AADD,
+		AADDW,
+		ASUB,
+		ASUBW,
+		ACMP,
+		ACMPW,
+		ACMN,
+		ACMNW:
+		if p.From.Type == D_CONST && p.From.Offset < 0 {
+			p.From.Offset = -p.From.Offset
+			p.As = complements[p.As]
+		}
+
+		break
+	}
 }
 
 func follow(ctxt *obj.Link, s *obj.LSym) {

src/liblink/asm7.c

@@ -911,7 +911,7 @@ addpool(Link *ctxt, Prog *p, Addr *a)
 	so we need to know what to do with C_MOVCON.
 	
 	The correct fix is to use the "negation" instruction variant,
-	e.g. CMN $0, R instead of CMP $-1, R, or SUB $1, R instead
+	e.g. CMN $1, R instead of CMP $-1, R, or SUB $1, R instead
 	of ADD $-1, R. */
 	case C_MOVCON:
 

src/liblink/obj7.c

@@ -102,6 +102,17 @@ settextflag(Prog *p, int f)
 	p->reg = f;
 }
 
+static short complements[] = {
+	[AADD]	ASUB,
+	[AADDW]	ASUBW,
+	[ASUB]	AADD,
+	[ASUBW]	AADDW,
+	[ACMP]	ACMN,
+	[ACMPW]	ACMNW,
+	[ACMN]	ACMP,
+	[ACMNW]	ACMPW,
+};
+
 static void
 progedit(Link *ctxt, Prog *p)
 {
@@ -156,6 +167,24 @@ progedit(Link *ctxt, Prog *p)
 		}
 		break;
 	}
+
+	// Rewrite negative immediates as positive immediates with
+	// complementary instruction.
+	switch(p->as) {
+	case AADD:
+	case AADDW:
+	case ASUB:
+	case ASUBW:
+	case ACMP:
+	case ACMPW:
+	case ACMN:
+	case ACMNW:
+		if(p->from.type == D_CONST && p->from.offset < 0) {
+			p->from.offset = -p->from.offset;
+			p->as = complements[p->as];
+		}
+		break;
+	}
 }
 
 static void