Revert "runtime, cmd: rationalize StackLimit and StackGuard"

This reverts commit CL 486380.

Submitted out of order and breaks bootstrap.

Change-Id: I67bd225094b5c9713b97f70feba04d2c99b7da76
Reviewed-on: https://go-review.googlesource.com/c/go/+/486916
Reviewed-by: David Chase <drchase@google.com>
TryBot-Bypass: Austin Clements <austin@google.com>

5 files changed, 24 insertions, 22 deletions

diff --git a/src/cmd/internal/objabi/stack.go b/src/cmd/internal/objabi/stack.go
index 7c7ff4e058..5a2f641a75 100644
--- a/src/cmd/internal/objabi/stack.go
+++ b/src/cmd/internal/objabi/stack.go
@@ -9,9 +9,18 @@ import (
 	"internal/buildcfg"
 )
 
-func StackNosplit(race bool) int {
-	// This arithmetic must match that in runtime/stack.go:stackNosplit.
-	return abi.StackNosplitBase * stackGuardMultiplier(race)
+// 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
 }
 
 // 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 24a96fb996..c82dafe51e 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.StackNosplit(*flagRace) - sc.callSize
+	limit := objabi.StackLimit(*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 8e3327ee48..9efd21b167 100644
--- a/src/internal/abi/stack.go
+++ b/src/internal/abi/stack.go
@@ -5,14 +5,6 @@
 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 e19e6d3d7a..a6623c0ec2 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 > stackNosplit {
+	if asyncPreemptStack > _StackLimit {
 		// 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 708a6ee2e5..39dbed5114 100644
--- a/src/runtime/stack.go
+++ b/src/runtime/stack.go
@@ -85,18 +85,19 @@ 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 a stackNosplit chain of NOSPLIT calls
-	// plus one stackSmall frame plus stackSystem bytes for the OS.
+	// 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.
 	// This arithmetic must match that in cmd/internal/objabi/stack.go:StackLimit.
-	_StackGuard = stackNosplit + _StackSystem + abi.StackSmall
+	_StackLimit = _StackGuard - _StackSystem - abi.StackSmall
 )
 
 const (
@@ -1210,7 +1211,7 @@ 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 + stackNosplit; used >= avail/4 {
+	if used := gp.stack.hi - gp.sched.sp + _StackLimit; used >= avail/4 {
 		return
 	}