fix(asm): encode amd64 frame adjustments above 127 bytes with imm32

asm/assemble_test.go

@@ -4,6 +4,7 @@
 package asm
 
 import (
+	"bytes"
 	"strings"
 	"testing"
 
@@ -351,3 +352,26 @@ TEXT ·pf(SB), NOSPLIT, $0
 		t.Errorf("PREFETCHT0 bytes: got %s, want 0f 18 0b", hex)
 	}
 }
+
+// TestSubSPEncodings pins the prologue SUB against the bytes go tool asm
+// emits for SUBQ $size, SP: imm8 for -128..127, the imm32 form for anything
+// larger.  The intermediate 129..255 range used to encode an ADD with a
+// truncated immediate, moving SP the wrong way.
+func TestSubSPEncodings(t *testing.T) {
+	for _, tt := range []struct {
+		size int
+		want []byte
+	}{
+		{8, []byte{0x48, 0x83, 0xEC, 0x08}},
+		{127, []byte{0x48, 0x83, 0xEC, 0x7F}},
+		{128, []byte{0x48, 0x81, 0xEC, 0x80, 0x00, 0x00, 0x00}},
+		{200, []byte{0x48, 0x81, 0xEC, 0xC8, 0x00, 0x00, 0x00}},
+		{255, []byte{0x48, 0x81, 0xEC, 0xFF, 0x00, 0x00, 0x00}},
+		{4096, []byte{0x48, 0x81, 0xEC, 0x00, 0x10, 0x00, 0x00}},
+	} {
+		got := subSP(tt.size)
+		if !bytes.Equal(got, tt.want) {
+			t.Errorf("subSP(%d) = %x, want %x", tt.size, got, tt.want)
+		}
+	}
+}