cmd/internal/obj/s390x: simplify huge frame prologue

CL 307010 for s390x.

Change-Id: I43e1f93dd01c814417f8ef7480aa82c05b2b6b66
Reviewed-on: https://go-review.googlesource.com/c/go/+/307151
Trust: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

diff --git a/src/cmd/internal/obj/s390x/objz.go b/src/cmd/internal/obj/s390x/objz.go
index a02c4fc17f482797478d7334d2ca9d1d3727538f..201163b0159a1883bde0570d9a516f6a49ab9113 100644 (file)
--- a/src/cmd/internal/obj/s390x/objz.go
+++ b/src/cmd/internal/obj/s390x/objz.go
@@ -568,7 +568,6 @@ func preprocess(ctxt *obj.Link, cursym *obj.LSym, newprog obj.ProgAlloc) {
 }
 
 func (c *ctxtz) stacksplitPre(p *obj.Prog, framesize int32) (*obj.Prog, *obj.Prog) {
-       var q *obj.Prog
 
        // MOVD g_stackguard(g), R3
        p = obj.Appendp(p, c.newprog)
@@ -589,98 +588,69 @@ func (c *ctxtz) stacksplitPre(p *obj.Prog, framesize int32) (*obj.Prog, *obj.Pro
        // unnecessarily. See issue #35470.
        p = c.ctxt.StartUnsafePoint(p, c.newprog)
 
-       q = nil
        if framesize <= objabi.StackSmall {
                // small stack: SP < stackguard
                //      CMPUBGE stackguard, SP, label-of-call-to-morestack
 
                p = obj.Appendp(p, c.newprog)
-               //q1 = p
                p.From.Type = obj.TYPE_REG
                p.From.Reg = REG_R3
                p.Reg = REGSP
                p.As = ACMPUBGE
                p.To.Type = obj.TYPE_BRANCH
 
-       } else if framesize <= objabi.StackBig {
-               // large stack: SP-framesize < stackguard-StackSmall
-               //      ADD $-(framesize-StackSmall), SP, R4
-               //      CMPUBGE stackguard, R4, label-of-call-to-morestack
-               p = obj.Appendp(p, c.newprog)
+               return p, nil
+       }
 
-               p.As = AADD
-               p.From.Type = obj.TYPE_CONST
-               p.From.Offset = -(int64(framesize) - objabi.StackSmall)
-               p.Reg = REGSP
-               p.To.Type = obj.TYPE_REG
-               p.To.Reg = REG_R4
+       // large stack: SP-framesize < stackguard-StackSmall
 
-               p = obj.Appendp(p, c.newprog)
-               p.From.Type = obj.TYPE_REG
-               p.From.Reg = REG_R3
-               p.Reg = REG_R4
-               p.As = ACMPUBGE
-               p.To.Type = obj.TYPE_BRANCH
-
-       } else {
-               // Such a large stack we need to protect against wraparound.
-               // If SP is close to zero:
-               //      SP-stackguard+StackGuard <= framesize + (StackGuard-StackSmall)
-               // The +StackGuard on both sides is required to keep the left side positive:
-               // SP is allowed to be slightly below stackguard. See stack.h.
+       var q *obj.Prog
+       offset := int64(framesize) - objabi.StackSmall
+       if framesize > objabi.StackBig {
+               // Such a large stack we need to protect against underflow.
+               // The runtime guarantees SP > objabi.StackBig, but
+               // framesize is large enough that SP-framesize may
+               // underflow, causing a direct comparison with the
+               // stack guard to incorrectly succeed. We explicitly
+               // guard against underflow.
                //
-               // Preemption sets stackguard to StackPreempt, a very large value.
-               // That breaks the math above, so we have to check for that explicitly.
-               //      // stackguard is R3
-               //      CMP     R3, $StackPreempt
-               //      BEQ     label-of-call-to-morestack
-               //      ADD     $StackGuard, SP, R4
-               //      SUB     R3, R4
-               //      MOVD    $(framesize+(StackGuard-StackSmall)), TEMP
-               //      CMPUBGE TEMP, R4, label-of-call-to-morestack
-               p = obj.Appendp(p, c.newprog)
-
-               p.As = ACMP
-               p.From.Type = obj.TYPE_REG
-               p.From.Reg = REG_R3
-               p.To.Type = obj.TYPE_CONST
-               p.To.Offset = objabi.StackPreempt
-
-               p = obj.Appendp(p, c.newprog)
-               q = p
-               p.As = ABEQ
-               p.To.Type = obj.TYPE_BRANCH
-
-               p = obj.Appendp(p, c.newprog)
-               p.As = AADD
-               p.From.Type = obj.TYPE_CONST
-               p.From.Offset = int64(objabi.StackGuard)
-               p.Reg = REGSP
-               p.To.Type = obj.TYPE_REG
-               p.To.Reg = REG_R4
-
-               p = obj.Appendp(p, c.newprog)
-               p.As = ASUB
-               p.From.Type = obj.TYPE_REG
-               p.From.Reg = REG_R3
-               p.To.Type = obj.TYPE_REG
-               p.To.Reg = REG_R4
+               //      MOVD    $(framesize-StackSmall), R4
+               //      CMPUBLT SP, R4, label-of-call-to-morestack
 
                p = obj.Appendp(p, c.newprog)
                p.As = AMOVD
                p.From.Type = obj.TYPE_CONST
-               p.From.Offset = int64(framesize) + int64(objabi.StackGuard) - objabi.StackSmall
+               p.From.Offset = offset
                p.To.Type = obj.TYPE_REG
-               p.To.Reg = REGTMP
+               p.To.Reg = REG_R4
 
                p = obj.Appendp(p, c.newprog)
+               q = p
+               p.As = ACMPUBLT
                p.From.Type = obj.TYPE_REG
-               p.From.Reg = REGTMP
+               p.From.Reg = REGSP
                p.Reg = REG_R4
-               p.As = ACMPUBGE
                p.To.Type = obj.TYPE_BRANCH
        }
 
+       // Check against the stack guard. We've ensured this won't underflow.
+       //      ADD $-(framesize-StackSmall), SP, R4
+       //      CMPUBGE stackguard, R4, label-of-call-to-morestack
+       p = obj.Appendp(p, c.newprog)
+       p.As = AADD
+       p.From.Type = obj.TYPE_CONST
+       p.From.Offset = -offset
+       p.Reg = REGSP
+       p.To.Type = obj.TYPE_REG
+       p.To.Reg = REG_R4
+
+       p = obj.Appendp(p, c.newprog)
+       p.From.Type = obj.TYPE_REG
+       p.From.Reg = REG_R3
+       p.Reg = REG_R4
+       p.As = ACMPUBGE
+       p.To.Type = obj.TYPE_BRANCH
+
        return p, q
 }