runtime, cmd: rationalize StackLimit and StackGuard

The current definitions of StackLimit and StackGuard only indirectly
specify the NOSPLIT stack limit and duplicate a literal constant
(928). Currently, they define the stack guard delta, and from there
compute the NOSPLIT limit.

Rationalize these by defining a new constant, abi.StackNosplitBase,
which consolidates and directly specifies the NOSPLIT stack limit (in
the default case). From this we then compute the stack guard delta,
inverting the relationship between these two constants. While we're
here, we rename StackLimit to StackNosplit to make it clearer what's
being limited.

This change does not affect the values of these constants in the
default configuration. It does slightly change how
StackGuardMultiplier values other than 1 affect the constants, but
this multiplier is a pretty rough heuristic anyway.

                    before after
stackNosplit           800   800
_StackGuard            928   928
stackNosplit -race    1728  1600
_StackGuard -race     1856  1728

For #59670.

Change-Id: Ia94094c5e47897e7c088d24b4a5e33f5c2768db5
Reviewed-on: https://go-review.googlesource.com/c/go/+/486976
Auto-Submit: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

5 files changed, 22 insertions, 24 deletions

diff --git a/src/cmd/internal/objabi/stack.go b/src/cmd/internal/objabi/stack.go
index 5a2f641a75..7c7ff4e058 100644
--- a/src/cmd/internal/objabi/stack.go
+++ b/src/cmd/internal/objabi/stack.go
@@ -9,18 +9,9 @@ import (
 	"internal/buildcfg"
 )
 
-// For the linkers. Must match Go definitions.
-
-const (
-	STACKSYSTEM = 0
-	StackSystem = STACKSYSTEM
-)
-
-func StackLimit(race bool) int {
-	// This arithmetic must match that in runtime/stack.go:{_StackGuard,_StackLimit}.
-	stackGuard := 928*stackGuardMultiplier(race) + StackSystem
-	stackLimit := stackGuard - StackSystem - abi.StackSmall
-	return stackLimit
+func StackNosplit(race bool) int {
+	// This arithmetic must match that in runtime/stack.go:stackNosplit.
+	return abi.StackNosplitBase * stackGuardMultiplier(race)
 }
 
 // stackGuardMultiplier returns a multiplier to apply to the default
diff --git a/src/cmd/link/internal/ld/stackcheck.go b/src/cmd/link/internal/ld/stackcheck.go
index c82dafe51e..24a96fb996 100644
--- a/src/cmd/link/internal/ld/stackcheck.go
+++ b/src/cmd/link/internal/ld/stackcheck.go
@@ -61,7 +61,7 @@ func (ctxt *Link) doStackCheck() {
 	// The call to morestack in every splittable function ensures
 	// that there are at least StackLimit bytes available below SP
 	// when morestack returns.
-	limit := objabi.StackLimit(*flagRace) - sc.callSize
+	limit := objabi.StackNosplit(*flagRace) - sc.callSize
 	if buildcfg.GOARCH == "arm64" {
 		// Need an extra 8 bytes below SP to save FP.
 		limit -= 8
diff --git a/src/internal/abi/stack.go b/src/internal/abi/stack.go
index 9efd21b167..8e3327ee48 100644
--- a/src/internal/abi/stack.go
+++ b/src/internal/abi/stack.go
@@ -5,6 +5,14 @@
 package abi
 
 const (
+	// StackNosplitBase is the base maximum number of bytes that a chain of
+	// NOSPLIT functions can use.
+	//
+	// This value must be multiplied by the stack guard multiplier, so do not
+	// use it directly. See runtime/stack.go:stackNosplit and
+	// cmd/internal/objabi/stack.go:StackNosplit.
+	StackNosplitBase = 800
+
 	// We have three different sequences for stack bounds checks, depending on
 	// whether the stack frame of a function is small, big, or huge.
 
diff --git a/src/runtime/preempt.go b/src/runtime/preempt.go
index a575fbcf11..38114243bb 100644
--- a/src/runtime/preempt.go
+++ b/src/runtime/preempt.go
@@ -320,7 +320,7 @@ func init() {
 	total += funcMaxSPDelta(f)
 	// Add some overhead for return PCs, etc.
 	asyncPreemptStack = uintptr(total) + 8*goarch.PtrSize
-	if asyncPreemptStack > _StackLimit {
+	if asyncPreemptStack > stackNosplit {
 		// We need more than the nosplit limit. This isn't
 		// unsafe, but it may limit asynchronous preemption.
 		//
diff --git a/src/runtime/stack.go b/src/runtime/stack.go
index 6d75301556..92c2ce3cbf 100644
--- a/src/runtime/stack.go
+++ b/src/runtime/stack.go
@@ -85,19 +85,18 @@ const (
 	_FixedStack6 = _FixedStack5 | (_FixedStack5 >> 16)
 	_FixedStack  = _FixedStack6 + 1
 
+	// stackNosplit is the maximum number of bytes that a chain of NOSPLIT
+	// functions can use.
+	// This arithmetic must match that in cmd/internal/objabi/stack.go:StackNosplit.
+	stackNosplit = abi.StackNosplitBase * sys.StackGuardMultiplier
+
 	// The stack guard is a pointer this many bytes above the
 	// bottom of the stack.
 	//
-	// The guard leaves enough room for one _StackSmall frame plus
-	// a _StackLimit chain of NOSPLIT calls plus _StackSystem
-	// bytes for the OS.
-	// This arithmetic must match that in cmd/internal/objabi/stack.go:StackLimit.
-	_StackGuard = 928*sys.StackGuardMultiplier + _StackSystem
-
-	// The maximum number of bytes that a chain of NOSPLIT
-	// functions can use.
+	// The guard leaves enough room for a stackNosplit chain of NOSPLIT calls
+	// plus one stackSmall frame plus stackSystem bytes for the OS.
 	// This arithmetic must match that in cmd/internal/objabi/stack.go:StackLimit.
-	_StackLimit = _StackGuard - _StackSystem - abi.StackSmall
+	_StackGuard = stackNosplit + _StackSystem + abi.StackSmall
 )
 
 const (
@@ -1211,7 +1210,7 @@ func shrinkstack(gp *g) {
 	// down to the SP plus the stack guard space that ensures
 	// there's room for nosplit functions.
 	avail := gp.stack.hi - gp.stack.lo
-	if used := gp.stack.hi - gp.sched.sp + _StackLimit; used >= avail/4 {
+	if used := gp.stack.hi - gp.sched.sp + stackNosplit; used >= avail/4 {
 		return
 	}