cmd/link: add Minalign field to ld.Arch

The new Minalign field sets the minimum alignment for all symbols.
This is required for the upcoming s390x port which requires symbols
be 2-byte aligned for efficient relative addressing.

All preexisting architectures have Minalign set to 1 which means
that this commit should have no effect.

I tested values of 2, 4 and 8 on linux amd64 and the tests appear to
pass. Increasing Minalign to 16 appears to break the runtime. I
think this is due to assumptions made about the layout of module
data.

toolstash -cmp on linux amd64 shows no changes due to this commit.

Resolves #14604

Change-Id: I0fe042d52c4e4732eba5fabcd0c31102a2408764
Reviewed-on: https://go-review.googlesource.com/20149
Reviewed-by: Minux Ma <minux@golang.org>
Run-TryBot: Minux Ma <minux@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/link/internal/amd64/l.go

@@ -33,6 +33,7 @@ package amd64
 const (
 	thechar   = '6'
 	MaxAlign  = 32 // max data alignment
+	MinAlign  = 1  // min data alignment
 	FuncAlign = 16
 )
 

src/cmd/link/internal/amd64/obj.go

@@ -57,6 +57,7 @@ func linkarchinit() {
 	ld.Thearch.Regsize = ld.Thelinkarch.Regsize
 	ld.Thearch.Funcalign = FuncAlign
 	ld.Thearch.Maxalign = MaxAlign
+	ld.Thearch.Minalign = MinAlign
 	ld.Thearch.Minlc = MINLC
 	ld.Thearch.Dwarfregsp = DWARFREGSP
 	ld.Thearch.Dwarfreglr = DWARFREGLR

src/cmd/link/internal/arm/l.go

@@ -65,6 +65,7 @@ package arm
 const (
 	thechar   = '5'
 	MaxAlign  = 8 // max data alignment
+	MinAlign  = 1 // min data alignment
 	FuncAlign = 4 // single-instruction alignment
 	MINLC     = 4
 )

src/cmd/link/internal/arm/obj.go

@@ -54,6 +54,7 @@ func linkarchinit() {
 	ld.Thearch.Regsize = ld.Thelinkarch.Regsize
 	ld.Thearch.Funcalign = FuncAlign
 	ld.Thearch.Maxalign = MaxAlign
+	ld.Thearch.Minalign = MinAlign
 	ld.Thearch.Minlc = MINLC
 	ld.Thearch.Dwarfregsp = DWARFREGSP
 	ld.Thearch.Dwarfreglr = DWARFREGLR

src/cmd/link/internal/arm64/l.go

@@ -64,6 +64,7 @@ package arm64
 const (
 	thechar   = '7'
 	MaxAlign  = 32 // max data alignment
+	MinAlign  = 1  // min data alignment
 	FuncAlign = 8
 	MINLC     = 4
 )

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

@@ -54,6 +54,7 @@ func linkarchinit() {
 	ld.Thearch.Regsize = ld.Thelinkarch.Regsize
 	ld.Thearch.Funcalign = FuncAlign
 	ld.Thearch.Maxalign = MaxAlign
+	ld.Thearch.Minalign = MinAlign
 	ld.Thearch.Minlc = MINLC
 	ld.Thearch.Dwarfregsp = DWARFREGSP
 	ld.Thearch.Dwarfreglr = DWARFREGLR

src/cmd/link/internal/ld/data.go

@@ -1026,18 +1026,18 @@ func dosymtype() {
 	}
 }
 
+// symalign returns the required alignment for the given symbol s.
 func symalign(s *LSym) int32 {
-	if s.Align != 0 {
+	min := int32(Thearch.Minalign)
+	if s.Align >= min {
 		return s.Align
+	} else if s.Align != 0 {
+		return min
 	}
-
 	align := int32(Thearch.Maxalign)
-	for int64(align) > s.Size && align > 1 {
+	for int64(align) > s.Size && align > min {
 		align >>= 1
 	}
-	if align < s.Align {
-		align = s.Align
-	}
 	return align
 }
 

src/cmd/link/internal/ld/lib.go

@@ -88,6 +88,7 @@ type Arch struct {
 	Regsize          int
 	Funcalign        int
 	Maxalign         int
+	Minalign         int
 	Minlc            int
 	Dwarfregsp       int
 	Dwarfreglr       int

src/cmd/link/internal/mips64/l.go

@@ -64,6 +64,7 @@ package mips64
 const (
 	thechar   = '0'
 	MaxAlign  = 32 // max data alignment
+	MinAlign  = 1  // min data alignment
 	FuncAlign = 8
 	MINLC     = 4
 )

src/cmd/link/internal/mips64/obj.go

@@ -58,6 +58,7 @@ func linkarchinit() {
 	ld.Thearch.Regsize = ld.Thelinkarch.Regsize
 	ld.Thearch.Funcalign = FuncAlign
 	ld.Thearch.Maxalign = MaxAlign
+	ld.Thearch.Minalign = MinAlign
 	ld.Thearch.Minlc = MINLC
 	ld.Thearch.Dwarfregsp = DWARFREGSP
 	ld.Thearch.Dwarfreglr = DWARFREGLR

src/cmd/link/internal/ppc64/l.go

@@ -64,6 +64,7 @@ package ppc64
 const (
 	thechar   = '9'
 	MaxAlign  = 32 // max data alignment
+	MinAlign  = 1  // min data alignment
 	FuncAlign = 8
 	MINLC     = 4
 )

src/cmd/link/internal/ppc64/obj.go

@@ -58,6 +58,7 @@ func linkarchinit() {
 	ld.Thearch.Regsize = ld.Thelinkarch.Regsize
 	ld.Thearch.Funcalign = FuncAlign
 	ld.Thearch.Maxalign = MaxAlign
+	ld.Thearch.Minalign = MinAlign
 	ld.Thearch.Minlc = MINLC
 	ld.Thearch.Dwarfregsp = DWARFREGSP
 	ld.Thearch.Dwarfreglr = DWARFREGLR

src/cmd/link/internal/x86/l.go

@@ -34,6 +34,7 @@ const (
 	thechar   = '8'
 	PtrSize   = 4
 	MaxAlign  = 32 // max data alignment
+	MinAlign  = 1  // min data alignment
 	FuncAlign = 16
 	MINLC     = 1
 )

src/cmd/link/internal/x86/obj.go

@@ -54,6 +54,7 @@ func linkarchinit() {
 	ld.Thearch.Regsize = ld.Thelinkarch.Regsize
 	ld.Thearch.Funcalign = FuncAlign
 	ld.Thearch.Maxalign = MaxAlign
+	ld.Thearch.Minalign = MinAlign
 	ld.Thearch.Minlc = MINLC
 	ld.Thearch.Dwarfregsp = DWARFREGSP
 	ld.Thearch.Dwarfreglr = DWARFREGLR