feat(asm): PCALIGN alignment on amd64

Assisted-by: GLM 5.3 Flash

CHANGELOG.md

@@ -22,13 +22,14 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   relocation, per-GOOS); arm64 accepts the bare-register indirect branch
   (`BL R9` beside `BL (R9)`, both BLR) and the zero-immediate store
   (`MOVD $0, mem` through the zero register, rejecting non-zero immediates
-  as the toolchain does); and `gasm asm` predefines the `GOARCH_<arch>`
-  and `GOOS_<goos>` macros the go command passes to `go tool asm`, so
-  GOROOT headers' `#ifdef GOARCH_amd64` platform blocks (`go_tls.h`'s
-  `get_tls` and friends) select as intended.  The GOROOT corpus measure
-  moves to 276 of 353 files assembling for every target architecture
-  (78.2 %), 91.4 % of the real-code corpus (280 of 303), from 70.8 %
-  and 82.2 %.
+  as the toolchain does); `PCALIGN` now aligns on amd64, padding with the
+  toolchain's greedy single-instruction NOPs; and `gasm asm` predefines
+  the `GOARCH_<arch>` and `GOOS_<goos>` macros the go command passes to
+  `go tool asm`, so GOROOT headers' `#ifdef GOARCH_amd64` platform blocks
+  (`go_tls.h`'s `get_tls` and friends) select as intended.  The GOROOT
+  corpus measure moves to 285 of 353 files assembling for every target
+  architecture (80.7 %), 91.4 % of the real-code corpus (280 of 303),
+  from 70.8 % and 82.2 %.
 
 ### Added
 

asm/assemble.go

@@ -122,6 +122,19 @@ func assemble(t *ast.Text, link *linkInfo) ([]byte, []sbPatch, map[string]int, [
 			case *ast.Label:
 				offsets[s.Name.Text] = pos
 			case *ast.Instr:
+				if strings.ToUpper(s.Mnemonic.Text) == "PCALIGN" {
+					// The alignment pseudo-statement: its size is the
+					// padding to the next boundary at this very position,
+					// filled with NOPs at emission.
+					pad, err := pcAlignPad(pcAlignValue(s), pos)
+					if err != nil {
+						return nil, nil, nil, nil, nil, nil, fmt.Errorf("PCALIGN: %w", err)
+					}
+					sizes[i] = pad
+					pcs[i] = pos
+					pos += pad
+					continue
+				}
 				sz, err := instrSize(s, fi, long[i], link)
 				if err != nil {
 					return nil, nil, nil, nil, nil, nil, fmt.Errorf("%s: %w", s.Mnemonic.Text, err)
@@ -548,6 +561,52 @@ func pcJumpTarget(t *ast.Text, j, n int, pcs []int) (int, bool) {
 	return 0, false
 }
 
+// x86 NOP encodings, single-instruction no-ops of lengths 1 to 9 (the
+// toolchain's asm6.go nop table); longer padding repeats the largest that
+// fits, greedy from the end.
+var x86Nops = [][]byte{
+	{0x90},
+	{0x66, 0x90},
+	{0x0F, 0x1F, 0x00},
+	{0x0F, 0x1F, 0x40, 0x00},
+	{0x0F, 0x1F, 0x44, 0x00, 0x00},
+	{0x66, 0x0F, 0x1F, 0x44, 0x00, 0x00},
+	{0x0F, 0x1F, 0x80, 0x00, 0x00, 0x00, 0x00},
+	{0x0F, 0x1F, 0x84, 0x00, 0x00, 0x00, 0x00, 0x00},
+	{0x66, 0x0F, 0x1F, 0x84, 0x00, 0x00, 0x00, 0x00, 0x00},
+}
+
+// fillNOPs fills p with the greedy largest single-instruction NOPs, exactly
+// the toolchain's fillnop.
+func fillNOPs(p []byte) {
+	for len(p) > 0 {
+		m := min(len(p), len(x86Nops))
+		copy(p[:m], x86Nops[m-1])
+		p = p[m:]
+	}
+}
+
+// pcAlignPad computes the padding PCALIGN $align inserts at pos: the
+// alignment must be a power of two in [8, 2048] and the padding runs to the
+// next boundary (zero when the position is already aligned).
+func pcAlignPad(align, pos int) (int, error) {
+	if align <= 0 || align&(align-1) != 0 || align < 8 || align > 2048 {
+		return 0, fmt.Errorf("alignment value of an instruction must be a power of two and in the range [8, 2048], got %d", align)
+	}
+	if lob := pos & (align - 1); lob != 0 {
+		return align - lob, nil
+	}
+	return 0, nil
+}
+
+// pcAlignValue reads a PCALIGN statement's alignment operand.
+func pcAlignValue(s *ast.Instr) int {
+	if len(s.Operands) == 1 && s.Operands[0].Kind == ast.OpImmediate && s.Operands[0].Imm.HasVal {
+		return int(s.Operands[0].Imm.Val)
+	}
+	return 0 // rejected by pcAlignPad's range check
+}
+
 // hasCall reports whether the function body contains a CALL instruction.
 func hasCall(t *ast.Text) bool {
 	for _, stmt := range t.Body {
@@ -760,6 +819,18 @@ func jumpSize(mnem string, long bool) int {
 func encodeInstr(s *ast.Instr, pc int, offsets map[string]int, fi frameInfo, long bool, resolve func(string) string, link *linkInfo, numTarget int) ([]byte, []sbPatch, []floatPoolEntry, error) {
 	mnem := strings.ToUpper(s.Mnemonic.Text)
 
+	if mnem == "PCALIGN" {
+		// The layout pass already accounted the padding; emit the same
+		// amount of NOP bytes for the statement's own position.
+		pad, err := pcAlignPad(pcAlignValue(s), pc)
+		if err != nil {
+			return nil, nil, nil, err
+		}
+		out := make([]byte, pad)
+		fillNOPs(out)
+		return out, nil, nil, nil
+	}
+
 	var prefix []byte
 	if mnem == "RET" && fi.useFP {
 		prefix = fi.epilogue

testdata/verify/forms_amd64.s

@@ -41,3 +41,12 @@ TEXT ·Tls(SB), NOSPLIT, $0-8
 	MOVQ	0(BX)(TLS*1), AX
 	MOVQ	AX, ret+0(FP)
 	RET
+
+// func Aligned() int64
+TEXT ·Aligned(SB), NOSPLIT, $0-8
+	MOVQ	$1, AX
+	PCALIGN	$16
+	MOVQ	$2, AX
+	PCALIGN	$32
+	MOVQ	AX, ret+0(FP)
+	RET