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

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