runtime: arm5, fix large-offset floating-point stores

The code sequence for large-offset floating-point stores
includes adding the base pointer to r11.  Make sure we
can interpret that instruction correctly.

Fixes build.

Fixes #15440

Change-Id: I7fe5a4a57e08682967052bf77c54e0ec47fcb53e
Reviewed-on: https://go-review.googlesource.com/22440Reviewed-by: Michael Hudson-Doyle <michael.hudson@canonical.com>

src/runtime/softfloat_arm.go

@@ -168,14 +168,15 @@ execute:
 		}
 		return 1
 	}
-	if i == 0xe08bb00d {
-		// add sp to r11.
-		// might be part of a large stack offset address
+	if i&0xfffffff0 == 0xe08bb000 {
+		r := i & 0xf
+		// add r to r11.
+		// might be part of a large offset address calculation
 		// (or might not, but again no harm done).
-		regs[11] += regs[13]
+		regs[11] += regs[r]
 
 		if fptrace > 0 {
-			print("*** cpu R[11] += R[13] ", hex(regs[11]), "\n")
+			print("*** cpu R[11] += R[", r, "] ", hex(regs[11]), "\n")
 		}
 		return 1
 	}

src/runtime/vlop_arm_test.go

@@ -82,3 +82,47 @@ func TestUsplit(t *testing.T) {
 		}
 	}
 }
+
+//go:noinline
+func armFloatWrite(a *[129]float64) {
+	// This used to miscompile on arm5.
+	// The offset is too big to fit in a load.
+	// So the code does:
+	//   ldr     r0, [sp, #8]
+	//   bl      6f690 <_sfloat>
+	//   ldr     fp, [pc, #32]   ; (address of 128.0)
+	//   vldr    d0, [fp]
+	//   ldr     fp, [pc, #28]   ; (1024)
+	//   add     fp, fp, r0
+	//   vstr    d0, [fp]
+	// The software floating-point emulator gives up on the add.
+	// This causes the store to not work.
+	// See issue 15440.
+	a[128] = 128.0
+}
+func TestArmFloatBigOffsetWrite(t *testing.T) {
+	var a [129]float64
+	for i := 0; i < 128; i++ {
+		a[i] = float64(i)
+	}
+	armFloatWrite(&a)
+	for i, x := range a {
+		if x != float64(i) {
+			t.Errorf("bad entry %d:%f\n", i, x)
+		}
+	}
+}
+
+//go:noinline
+func armFloatRead(a *[129]float64) float64 {
+	return a[128]
+}
+func TestArmFloatBigOffsetRead(t *testing.T) {
+	var a [129]float64
+	for i := 0; i < 129; i++ {
+		a[i] = float64(i)
+	}
+	if x := armFloatRead(&a); x != 128.0 {
+		t.Errorf("bad value %f\n", x)
+	}
+}