cmd/internal/obj/ppc64: improve large prologue generation

Avoid needing an unsafe section to store LR and adjust SP
for large constants by using the stdux (MOVDU) instruction.

This is also a few instructions shorter as the large
constant adjustment is only created once.

Change-Id: I6ff7a24181cdadb1846a33129fc148dcf59b76d5
Reviewed-on: https://go-review.googlesource.com/c/go/+/710197
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Auto-Submit: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

diff --git a/src/cmd/internal/obj/ppc64/obj9.go b/src/cmd/internal/obj/ppc64/obj9.go
index 5615b70aad4f0e4f2ce0acd1c93d69af2d1afda2..bae2965639721f8155529a155068c2217e599145 100644 (file)
--- a/src/cmd/internal/obj/ppc64/obj9.go
+++ b/src/cmd/internal/obj/ppc64/obj9.go
@@ -903,44 +903,38 @@ func preprocess(ctxt *obj.Link, cursym *obj.LSym, newprog obj.ProgAlloc) {
                                        q.To.Reg = REGSP
                                        q.Spadj = autosize
                                } else {
-                                       // Frame size is too large for a MOVDU instruction.
-                                       // Store link register before decrementing SP, so if a signal comes
-                                       // during the execution of the function prologue, the traceback
-                                       // code will not see a half-updated stack frame.
-                                       // This sequence is not async preemptible, as if we open a frame
-                                       // at the current SP, it will clobber the saved LR.
+                                       // Frame size is too large for an stdu MOVDU instruction, use stdux MOVDU.
                                        q = obj.Appendp(q, c.newprog)
                                        q.As = AMOVD
                                        q.Pos = p.Pos
                                        q.From.Type = obj.TYPE_REG
                                        q.From.Reg = REG_LR
                                        q.To.Type = obj.TYPE_REG
-                                       q.To.Reg = REG_R29 // REGTMP may be used to synthesize large offset in the next instruction
-
-                                       q = c.ctxt.StartUnsafePoint(q, c.newprog)
+                                       q.To.Reg = REG_R29
 
+                                       // Create stack adjustment in REGTMP
                                        q = obj.Appendp(q, c.newprog)
                                        q.As = AMOVD
                                        q.Pos = p.Pos
-                                       q.From.Type = obj.TYPE_REG
-                                       q.From.Reg = REG_R29
-                                       q.To.Type = obj.TYPE_MEM
-                                       q.To.Offset = int64(-autosize)
-                                       q.To.Reg = REGSP
+                                       q.From.Type = obj.TYPE_CONST
+                                       q.From.Offset = int64(-autosize)
+                                       q.To.Type = obj.TYPE_REG
+                                       q.To.Reg = REGTMP
 
                                        prologueEnd = q
 
+                                       // MOVDU R29, R31(R1)
                                        q = obj.Appendp(q, c.newprog)
-                                       q.As = AADD
+                                       q.As = AMOVDU
                                        q.Pos = p.Pos
-                                       q.From.Type = obj.TYPE_CONST
-                                       q.From.Offset = int64(-autosize)
-                                       q.To.Type = obj.TYPE_REG
-                                       q.To.Reg = REGSP
-                                       q.Spadj = +autosize
-
-                                       q = c.ctxt.EndUnsafePoint(q, c.newprog, -1)
+                                       q.From.Type = obj.TYPE_REG
+                                       q.From.Reg = REG_R29
+                                       q.To.Type = obj.TYPE_MEM
+                                       q.To.Reg = REGTMP
+                                       q.To.Index = REGSP
+                                       q.Spadj = autosize
                                }
+
                                prologueEnd.Pos = prologueEnd.Pos.WithXlogue(src.PosPrologueEnd)
                        } else if c.cursym.Func().Text.Mark&LEAF == 0 {
                                // A very few functions that do not return to their caller