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

CL 307010 for arm.

Change-Id: I14d939eb8aa6f594927054a2595f8c270a0b607f
Reviewed-on: https://go-review.googlesource.com/c/go/+/307049
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/arm/a.out.go b/src/cmd/internal/obj/arm/a.out.go
index a1d9e28b96045ebe5f04910a6d27769a57a5da34..fd695ad0c98d1bbe8c4d984f1b7895a7b464ab35 100644 (file)
--- a/src/cmd/internal/obj/arm/a.out.go
+++ b/src/cmd/internal/obj/arm/a.out.go
@@ -163,8 +163,8 @@ const (
        C_SFCON
        C_LFCON
 
-       C_RACON
-       C_LACON
+       C_RACON /* <=0xff rotated constant offset from auto */
+       C_LACON /* Large Auto CONstant, i.e. large offset from SP */
 
        C_SBRA
        C_LBRA

diff --git a/src/cmd/internal/obj/arm/obj5.go b/src/cmd/internal/obj/arm/obj5.go
index 7de04302d918fe7d0d42b24f2172dcf3d3df75f9..edb384806b4bc1ed1be6f0b710d830a08231f8a8 100644 (file)
--- a/src/cmd/internal/obj/arm/obj5.go
+++ b/src/cmd/internal/obj/arm/obj5.go
@@ -680,57 +680,37 @@ func (c *ctxt5) stacksplit(p *obj.Prog, framesize int32) *obj.Prog {
                p.From.Reg = REG_R1
                p.Reg = REG_R2
        } 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.
-               //      CMP     $StackPreempt, R1
-               //      MOVW.NE $StackGuard(SP), R2
-               //      SUB.NE  R1, R2
-               //      MOVW.NE $(framesize+(StackGuard-StackSmall)), R3
-               //      CMP.NE  R3, R2
-               p = obj.Appendp(p, c.newprog)
-
-               p.As = ACMP
-               p.From.Type = obj.TYPE_CONST
-               p.From.Offset = int64(uint32(objabi.StackPreempt & (1<<32 - 1)))
-               p.Reg = REG_R1
-
-               p = obj.Appendp(p, c.newprog)
-               p.As = AMOVW
-               p.From.Type = obj.TYPE_ADDR
-               p.From.Reg = REGSP
-               p.From.Offset = int64(objabi.StackGuard)
-               p.To.Type = obj.TYPE_REG
-               p.To.Reg = REG_R2
-               p.Scond = C_SCOND_NE
+               // 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.
+               //
+               //      // Try subtracting from SP and check for underflow.
+               //      // If this underflows, it sets C to 0.
+               //      SUB.S $(framesize-StackSmall), SP, R2
+               //      // If C is 1 (unsigned >=), compare with guard.
+               //      CMP.HS stackguard, R2
 
                p = obj.Appendp(p, c.newprog)
                p.As = ASUB
-               p.From.Type = obj.TYPE_REG
-               p.From.Reg = REG_R1
+               p.Scond = C_SBIT
+               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_R2
-               p.Scond = C_SCOND_NE
-
-               p = obj.Appendp(p, c.newprog)
-               p.As = AMOVW
-               p.From.Type = obj.TYPE_ADDR
-               p.From.Offset = int64(framesize) + (int64(objabi.StackGuard) - objabi.StackSmall)
-               p.To.Type = obj.TYPE_REG
-               p.To.Reg = REG_R3
-               p.Scond = C_SCOND_NE
 
                p = obj.Appendp(p, c.newprog)
                p.As = ACMP
+               p.Scond = C_SCOND_HS
                p.From.Type = obj.TYPE_REG
-               p.From.Reg = REG_R3
+               p.From.Reg = REG_R1
                p.Reg = REG_R2
-               p.Scond = C_SCOND_NE
        }
 
-       // BLS call-to-morestack
+       // BLS call-to-morestack (C is 0 or Z is 1)
        bls := obj.Appendp(p, c.newprog)
        bls.As = ABLS
        bls.To.Type = obj.TYPE_BRANCH