fix(cmd): padding-aware ground-truth comparison

Assisted-by: GLM 5.3

cmd/gasm/main.go

@@ -938,11 +938,40 @@ func compareGroundTruth(img *asm.Image, gt map[string][]byte) (matched, total, d
 				goCmp[j] = 0
 			}
 		}
-		if bytes.Equal(gasmCmp, goCmp) {
+		// The toolchain pads text symbols to 16-byte boundaries with
+		// zeros, so a function whose size is not a multiple of 16
+		// carries trailing zeros in the ground truth that are not part
+		// of the encoding.  Compare up to the shorter side and require
+		// the remainder of whichever is longer to be zero, so padding
+		// never masks a real difference.
+		cmpLen := min(len(gasmCmp), len(goCmp))
+		equal := bytes.Equal(gasmCmp[:cmpLen], goCmp[:cmpLen])
+		if equal {
+			for _, b := range gasmCmp[cmpLen:] {
+				if b != 0 {
+					equal = false
+					break
+				}
+			}
+		}
+		if equal {
+			for _, b := range goCmp[cmpLen:] {
+				if b != 0 {
+					equal = false
+					break
+				}
+			}
+		}
+		if equal {
 			matched++
-			if len(fn.Relocs) > 0 {
+			switch {
+			case len(fn.Relocs) > 0 && len(goCmp) > cmpLen:
+				fmt.Printf("  %s: MATCH (%d bytes, %d relocs masked, %d padding)\n", fn.Name, fn.Size, len(fn.Relocs), len(goCmp)-cmpLen)
+			case len(fn.Relocs) > 0:
 				fmt.Printf("  %s: MATCH (%d bytes, %d relocs masked)\n", fn.Name, fn.Size, len(fn.Relocs))
-			} else {
+			case len(goCmp) > cmpLen:
+				fmt.Printf("  %s: MATCH (%d bytes, %d padding)\n", fn.Name, fn.Size, len(goCmp)-cmpLen)
+			default:
 				fmt.Printf("  %s: MATCH (%d bytes)\n", fn.Name, fn.Size)
 			}
 		} else {

cmd/gasm/main_test.go

@@ -15,6 +15,7 @@ import (
 	"testing"
 
 	"sourcedock.dev/petrbalvin/gasm-devkit/arch"
+	"sourcedock.dev/petrbalvin/gasm-devkit/asm"
 )
 
 const clean = "#include \"textflag.h\"\n" +
@@ -440,3 +441,22 @@ func TestRunCorpusAudit(t *testing.T) {
 		t.Errorf("arm64 unencodable reasons = %d, want 1", r)
 	}
 }
+
+// TestCompareGroundTruthPadding pins the padding-aware ground-truth
+// comparison: the toolchain pads text symbols to 16-byte boundaries, so
+// trailing zeros in the reference must not read as a mismatch, while any
+// non-zero tail still must.
+func TestCompareGroundTruthPadding(t *testing.T) {
+	code := []byte{0x48, 0x8b, 0x07, 0xc3} // 4 bytes, not a multiple of 16
+	img := &asm.Image{Code: code, Funcs: []asm.FuncLayout{{Name: "f", Offset: 0, Size: len(code)}}}
+	padded := append(append([]byte(nil), code...), 0, 0, 0)
+	matched, total, diffs := compareGroundTruth(img, map[string][]byte{"f": padded})
+	if matched != 1 || total != 1 || diffs != 0 {
+		t.Fatalf("zero padding should match: matched=%d total=%d diffs=%d", matched, total, diffs)
+	}
+	dirty := append(append([]byte(nil), code...), 0, 0x90, 0)
+	matched, _, diffs = compareGroundTruth(img, map[string][]byte{"f": dirty})
+	if matched != 0 || diffs != 1 {
+		t.Fatalf("non-zero padding must mismatch: matched=%d diffs=%d", matched, diffs)
+	}
+}