feat(asm): emit the riscv64 stack-split guard and fix large-frame addressing

asm/riscv_assemble.go

@@ -15,14 +15,16 @@ import (
 func assembleRISCV(t *ast.Text) ([]byte, map[string]int, []Reloc, []LineEntry, []SpadjStep, error) {
 	fi := riscvComputeFrame(t)
 	prologue := riscvPrologue(fi)
+	guardLen := riscvGuardLen(fi)
 
 	var relocs []Reloc
 	var spadj []SpadjStep
 
 	// The prologue raises the SP delta by autosize; the boundary is reported
 	// at the pc just past its ADDI, exactly as the toolchain's pctospadj does.
+	// The guard prefix shifts its PC.
 	if fi.autosize != 0 {
-		spadj = append(spadj, SpadjStep{PC: riscvPrologueSpadjPC(fi), Value: fi.autosize})
+		spadj = append(spadj, SpadjStep{PC: guardLen + riscvPrologueSpadjPC(fi), Value: fi.autosize})
 	}
 
 	// Pass 1: collect instructions and compute label offsets assuming 4 bytes
@@ -34,7 +36,7 @@ func assembleRISCV(t *ast.Text) ([]byte, map[string]int, []Reloc, []LineEntry, [
 	}
 	var recs []instrRec
 	offsets := map[string]int{}
-	pos := len(prologue)
+	pos := guardLen + len(prologue)
 	for _, stmt := range t.Body {
 		switch s := stmt.(type) {
 		case *ast.Label:
@@ -66,7 +68,7 @@ func assembleRISCV(t *ast.Text) ([]byte, map[string]int, []Reloc, []LineEntry, [
 
 	// Pass 4: recompute offsets with actual sizes.
 	offsets = map[string]int{}
-	pos = len(prologue)
+	pos = guardLen + len(prologue)
 	for _, stmt := range t.Body {
 		switch s := stmt.(type) {
 		case *ast.Label:
@@ -82,9 +84,17 @@ func assembleRISCV(t *ast.Text) ([]byte, map[string]int, []Reloc, []LineEntry, [
 	}
 
 	// Pass 5: re-encode branches with corrected offsets.  Record relocations
-	// during this final pass (relocation offsets are relative to instruction start).
-	out := append([]byte(nil), prologue...)
-	pc = len(prologue)
+	// during this final pass (relocation offsets are relative to instruction
+	// start).  The guard prefix precedes the prologue; its branches target
+	// the morestack block at the end of the function, which the previous
+	// passes have sized.
+	var out []byte
+	guardBytes, guardReloc := riscvGuard(fi)
+	if fi.needSplit {
+		out = append(out, guardBytes...)
+	}
+	out = append(out, prologue...)
+	pc = guardLen + len(prologue)
 	preCount := len(relocs)
 	var lines []LineEntry
 	for _, r := range recs {
@@ -119,6 +129,9 @@ func assembleRISCV(t *ast.Text) ([]byte, map[string]int, []Reloc, []LineEntry, [
 			pc += len(code)
 		}
 	}
+	if fi.needSplit {
+		relocs = append(relocs, guardReloc)
+	}
 	return out, offsets, relocs, lines, spadj, nil
 }