cmd/compile: redo arm64 LR/FP save and restore

Instead of storing LR (the return address) at 0(SP) and the FP
(parent's frame pointer) at -8(SP), store them at framesize-8(SP)
and framesize-16(SP), respectively.

We push and pop data onto the stack such that we're never accessing
anything below SP.

The prolog/epilog lengths are unchanged (3 insns for a typical prolog,
2 for a typical epilog).

We use 8 bytes more per frame.

Typical prologue:

    STP.W   (FP, LR), -16(SP)
    MOVD    SP, FP
    SUB	    $C, SP

Typical epilogue:

    ADD     $C, SP
    LDP.P   16(SP), (FP, LR)
    RET

The previous word where we stored LR, at 0(SP), is now unused.
We could repurpose that slot for storing a local variable.

The new prolog and epilog instructions are recognized by libunwind,
so pc-sampling tools like perf should now be accurate. (TODO: except
maybe after the first RET instruction? Have to look into that.)

Update #73753 (fixes, for arm64)
Update #57302 (Quim thinks this will help on that issue)

Change-Id: I4800036a9a9a08aaaf35d9f99de79a36cf37ebb8
Reviewed-on: https://go-review.googlesource.com/c/go/+/674615
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>

6 files changed, 40 insertions, 33 deletions

diff --git a/src/cmd/link/internal/amd64/obj.go b/src/cmd/link/internal/amd64/obj.go
index 3a6141b909..761496549f 100644
--- a/src/cmd/link/internal/amd64/obj.go
+++ b/src/cmd/link/internal/amd64/obj.go
@@ -51,15 +51,16 @@ func Init() (*sys.Arch, ld.Arch) {
 		Plan9Magic:  uint32(4*26*26 + 7),
 		Plan9_64Bit: true,
 
-		Adddynrel:        adddynrel,
-		Archinit:         archinit,
-		Archreloc:        archreloc,
-		Archrelocvariant: archrelocvariant,
-		Gentext:          gentext,
-		Machoreloc1:      machoreloc1,
-		MachorelocSize:   8,
-		PEreloc1:         pereloc1,
-		TLSIEtoLE:        tlsIEtoLE,
+		Adddynrel:                 adddynrel,
+		Archinit:                  archinit,
+		Archreloc:                 archreloc,
+		Archrelocvariant:          archrelocvariant,
+		Gentext:                   gentext,
+		Machoreloc1:               machoreloc1,
+		MachorelocSize:            8,
+		PEreloc1:                  pereloc1,
+		TLSIEtoLE:                 tlsIEtoLE,
+		ReturnAddressAtTopOfFrame: true,
 
 		ELF: ld.ELFArch{
 			Linuxdynld:     "/lib64/ld-linux-x86-64.so.2",
diff --git a/src/cmd/link/internal/arm64/obj.go b/src/cmd/link/internal/arm64/obj.go
index 3d358155ba..e1e4ade818 100644
--- a/src/cmd/link/internal/arm64/obj.go
+++ b/src/cmd/link/internal/arm64/obj.go
@@ -47,17 +47,18 @@ func Init() (*sys.Arch, ld.Arch) {
 		Dwarfreglr: dwarfRegLR,
 		TrampLimit: 0x7c00000, // 26-bit signed offset * 4, leave room for PLT etc.
 
-		Adddynrel:        adddynrel,
-		Archinit:         archinit,
-		Archreloc:        archreloc,
-		Archrelocvariant: archrelocvariant,
-		Extreloc:         extreloc,
-		Gentext:          gentext,
-		GenSymsLate:      gensymlate,
-		Machoreloc1:      machoreloc1,
-		MachorelocSize:   8,
-		PEreloc1:         pereloc1,
-		Trampoline:       trampoline,
+		Adddynrel:                 adddynrel,
+		Archinit:                  archinit,
+		Archreloc:                 archreloc,
+		Archrelocvariant:          archrelocvariant,
+		Extreloc:                  extreloc,
+		Gentext:                   gentext,
+		GenSymsLate:               gensymlate,
+		Machoreloc1:               machoreloc1,
+		MachorelocSize:            8,
+		PEreloc1:                  pereloc1,
+		Trampoline:                trampoline,
+		ReturnAddressAtTopOfFrame: true,
 
 		ELF: ld.ELFArch{
 			Androiddynld:   "/system/bin/linker64",
diff --git a/src/cmd/link/internal/ld/dwarf.go b/src/cmd/link/internal/ld/dwarf.go
index 0003938ef2..c4d12a5488 100644
--- a/src/cmd/link/internal/ld/dwarf.go
+++ b/src/cmd/link/internal/ld/dwarf.go
@@ -1544,9 +1544,14 @@ func (d *dwctxt) writeframes(fs loader.Sym) dwarfSecInfo {
 				if pcsp.Value > 0 {
 					// The return address is preserved at (CFA-frame_size)
 					// after a stack frame has been allocated.
+					off := -spdelta
+					if thearch.ReturnAddressAtTopOfFrame {
+						// Except arm64, which has it at the top of frame.
+						off = -int64(d.arch.PtrSize)
+					}
 					deltaBuf = append(deltaBuf, dwarf.DW_CFA_offset_extended_sf)
 					deltaBuf = dwarf.AppendUleb128(deltaBuf, uint64(thearch.Dwarfreglr))
-					deltaBuf = dwarf.AppendSleb128(deltaBuf, -spdelta/dataAlignmentFactor)
+					deltaBuf = dwarf.AppendSleb128(deltaBuf, off/dataAlignmentFactor)
 				} else {
 					// The return address is restored into the link register
 					// when a stack frame has been de-allocated.
diff --git a/src/cmd/link/internal/ld/lib.go b/src/cmd/link/internal/ld/lib.go
index 2c861129b5..5f5ebfc1d9 100644
--- a/src/cmd/link/internal/ld/lib.go
+++ b/src/cmd/link/internal/ld/lib.go
@@ -263,6 +263,10 @@ type Arch struct {
 	// optional override for assignAddress
 	AssignAddress func(ldr *loader.Loader, sect *sym.Section, n int, s loader.Sym, va uint64, isTramp bool) (*sym.Section, int, uint64)
 
+	// Reports whether the return address is stored at the top (highest address)
+	// of the stack frame.
+	ReturnAddressAtTopOfFrame bool
+
 	// ELF specific information.
 	ELF ELFArch
 }
diff --git a/src/cmd/link/internal/ld/stackcheck.go b/src/cmd/link/internal/ld/stackcheck.go
index 98e7edaeb1..14cd3a2238 100644
--- a/src/cmd/link/internal/ld/stackcheck.go
+++ b/src/cmd/link/internal/ld/stackcheck.go
@@ -9,7 +9,6 @@ import (
 	"cmd/internal/objabi"
 	"cmd/link/internal/loader"
 	"fmt"
-	"internal/buildcfg"
 	"sort"
 	"strings"
 )
@@ -62,10 +61,6 @@ func (ctxt *Link) doStackCheck() {
 	// that there are at least StackLimit bytes available below SP
 	// when morestack returns.
 	limit := objabi.StackNosplit(*flagRace) - sc.callSize
-	if buildcfg.GOARCH == "arm64" {
-		// Need an extra 8 bytes below SP to save FP.
-		limit -= 8
-	}
 
 	// Compute stack heights without any back-tracking information.
 	// This will almost certainly succeed and we can simply
diff --git a/src/cmd/link/internal/x86/obj.go b/src/cmd/link/internal/x86/obj.go
index 4336f01ea3..a4885fde8f 100644
--- a/src/cmd/link/internal/x86/obj.go
+++ b/src/cmd/link/internal/x86/obj.go
@@ -50,13 +50,14 @@ func Init() (*sys.Arch, ld.Arch) {
 
 		Plan9Magic: uint32(4*11*11 + 7),
 
-		Adddynrel:        adddynrel,
-		Archinit:         archinit,
-		Archreloc:        archreloc,
-		Archrelocvariant: archrelocvariant,
-		Gentext:          gentext,
-		Machoreloc1:      machoreloc1,
-		PEreloc1:         pereloc1,
+		Adddynrel:                 adddynrel,
+		Archinit:                  archinit,
+		Archreloc:                 archreloc,
+		Archrelocvariant:          archrelocvariant,
+		Gentext:                   gentext,
+		Machoreloc1:               machoreloc1,
+		PEreloc1:                  pereloc1,
+		ReturnAddressAtTopOfFrame: true,
 
 		ELF: ld.ELFArch{
 			Linuxdynld:     "/lib/ld-linux.so.2",