cmd/internal/obj/ppc64: fix rebuilding of optab for asm tests

The end-to-end asm tests reinitialize the assembler using different
GOPPC64 values. This caused duplicate entries to amass from the
prefix and generated optab entries. This bug only affects the
asm end-to-end tests.

On reinitialization, optab contains the previous prefixedOptab
and optabGen entries, not just the initial values. Rework the
initialization to avoid the stale optab entries.

Change-Id: I310499915a5272ed0174ed8135d60788e6b4b716
Reviewed-on: https://go-review.googlesource.com/c/go/+/528316
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

2 files changed, 29 insertions, 9 deletions

diff --git a/src/cmd/internal/obj/ppc64/asm9.go b/src/cmd/internal/obj/ppc64/asm9.go
index c1207b01f9..4ce506cbf9 100644
--- a/src/cmd/internal/obj/ppc64/asm9.go
+++ b/src/cmd/internal/obj/ppc64/asm9.go
@@ -99,7 +99,11 @@ type Optab struct {
 //
 // Likewise, each slice of optab is dynamically sorted using the ocmp Sort interface
 // to arrange entries to minimize text size of each opcode.
-var optab = []Optab{
+//
+// optab is the sorted result of combining optabBase, optabGen, and prefixableOptab.
+var optab []Optab
+
+var optabBase = []Optab{
 	{as: obj.ATEXT, a1: C_LOREG, a6: C_TEXTSIZE, type_: 0, size: 0},
 	{as: obj.ATEXT, a1: C_LOREG, a3: C_LCON, a6: C_TEXTSIZE, type_: 0, size: 0},
 	{as: obj.ATEXT, a1: C_ADDR, a6: C_TEXTSIZE, type_: 0, size: 0},
@@ -1303,10 +1307,6 @@ func buildop(ctxt *obj.Link) {
 			entry.ispfx = true
 			entry.size = entry.pfxsize
 		}
-		// Use the legacy assembler function if none provided.
-		if entry.asmout == nil {
-			entry.asmout = asmout
-		}
 		prefixOptab = append(prefixOptab, entry.Optab)
 
 	}
@@ -1318,16 +1318,20 @@ func buildop(ctxt *obj.Link) {
 			}
 		}
 	}
+
+	// Append the generated entries, sort, and fill out oprange.
+	optab = make([]Optab, 0, len(optabBase)+len(optabGen)+len(prefixOptab))
+	optab = append(optab, optabBase...)
+	optab = append(optab, optabGen...)
+	optab = append(optab, prefixOptab...)
+	sort.Slice(optab, optabLess)
+
 	for i := range optab {
 		// Use the legacy assembler function if none provided.
 		if optab[i].asmout == nil {
 			optab[i].asmout = asmout
 		}
 	}
-	// Append the generated entries, sort, and fill out oprange.
-	optab = append(optab, optabGen...)
-	optab = append(optab, prefixOptab...)
-	sort.Slice(optab, optabLess)
 
 	for i := 0; i < len(optab); {
 		r := optab[i].as
diff --git a/src/cmd/internal/obj/ppc64/asm_test.go b/src/cmd/internal/obj/ppc64/asm_test.go
index 433df5c8aa..87d4156ef9 100644
--- a/src/cmd/internal/obj/ppc64/asm_test.go
+++ b/src/cmd/internal/obj/ppc64/asm_test.go
@@ -7,6 +7,7 @@ package ppc64
 import (
 	"bytes"
 	"fmt"
+	"internal/buildcfg"
 	"internal/testenv"
 	"math"
 	"os"
@@ -553,3 +554,18 @@ func TestAddrClassifier(t *testing.T) {
 		}
 	}
 }
+
+// The optab size should remain constant when reinitializing the PPC64 assembler backend.
+func TestOptabReinit(t *testing.T) {
+	buildcfg.GOOS = "linux"
+	buildcfg.GOARCH = "ppc64le"
+	buildcfg.GOPPC64 = 8
+	buildop(nil)
+	optabLen := len(optab)
+	buildcfg.GOPPC64 = 9
+	buildop(nil)
+	reinitOptabLen := len(optab)
+	if reinitOptabLen != optabLen {
+		t.Errorf("rerunning buildop changes optab size from %d to %d", optabLen, reinitOptabLen)
+	}
+}