From 2b2a72d54ebdcf1f3045dec929e5f6a223fb78f7 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Petr=20Balv=C3=ADn?= <opensource@petrbalvin.org>
Date: Wed, 7 Oct 2026 02:12:04 +0200
Subject: [PATCH] fix(asm): encode the arm64 bitfield aliases with their own
 opc

Assisted-by: GLM 5.3 Flash
---
 asm/arm64_assemble.go    |  5 ++++-
 asm/arm64_encode_test.go | 36 ++++++++++++++++++++++++++++++++++++
 2 files changed, 40 insertions(+), 1 deletion(-)

diff --git a/asm/arm64_assemble.go b/asm/arm64_assemble.go
index 5b88eba..dc6855b 100644
--- a/asm/arm64_assemble.go
+++ b/asm/arm64_assemble.go
@@ -3636,7 +3636,10 @@ func encodeARM64BitfieldAlias(mnem string, baseOp uint32, ops []*ast.Operand) ([
 	case "BFXIL", "BFXILW":
 		immr, imms = lsb, lsb+width-1
 	default: // BFI, SBFIZ, UBFIZ
-		immr, imms = (-lsb)%bits, width-1
+		// immr = (bits - lsb) mod bits, the toolchain's 64-r form with the
+		// zero lsb folding to zero (Go's % keeps the sign, so the operand
+		// order matters here).
+		immr, imms = (bits-lsb)%bits, width-1
 	}
 	return a64wordLE(baseOp | uint32(immr)<<16 | uint32(imms)<<10 | uint32(rn)<<5 | uint32(rd)), nil
 }
diff --git a/asm/arm64_encode_test.go b/asm/arm64_encode_test.go
index ef7c3af..c55635e 100644
--- a/asm/arm64_encode_test.go
+++ b/asm/arm64_encode_test.go
@@ -2186,3 +2186,39 @@ func TestArm64ZRNameNotNumber(t *testing.T) {
 	}
 }
 
+// TestArm64BitfieldAlias pins the bitfield alias encodings against
+// `go tool asm` words: BFI rides the BFM opc at every lsb (asm7.go case 43
+// routes ABFI to ABFM), SBFIZ keeps SBFM and UBFIZ keeps UBFM at the spelled
+// width.
+func TestArm64BitfieldAlias(t *testing.T) {
+	got := arm64Words(t,
+		"\tBFI $0, R1, $1, R2\n"+
+			"\tBFIW $0, R1, $1, R2\n"+
+			"\tBFI $4, R1, $4, R2\n"+
+			"\tSBFIZ $0, R1, $1, R2\n"+
+			"\tSBFIZW $0, R1, $1, R2\n"+
+			"\tUBFIZ $0, R1, $1, R2\n"+
+			"\tUBFIZW $0, R1, $1, R2\n"+
+			"\tUBFIZ $4, R1, $4, R2\n"+
+			"\tBFXIL $0, R1, $8, R2\n")
+	want := []uint32{
+		0xb3400022, // BFM X2, X1, #0, #0 (BFI lsb 0)
+		0x33000022, // BFM W2, W1, #0, #0 (BFIW lsb 0)
+		0xb37c0c22, // BFM X2, X1, #28, #3 (BFI lsb 4)
+		0x93400022, // SBFM X2, X1, #0, #0 (SBFIZ lsb 0)
+		0x13000022, // SBFM W2, W1, #0, #0 (SBFIZW lsb 0)
+		0xd3400022, // UBFM X2, X1, #0, #0 (UBFIZ lsb 0)
+		0x53000022, // UBFM W2, W1, #0, #0 (UBFIZW lsb 0)
+		0xd37c0c22, // UBFM X2, X1, #28, #3 (UBFIZ lsb 4)
+		0xb3401c22, // BFM X2, X1, #0, #7 (BFXIL lsb 0)
+		0xd65f03c0, // RET
+	}
+	if len(got) != len(want) {
+		t.Fatalf("word count = %d, want %d", len(got), len(want))
+	}
+	for i := range want {
+		if got[i] != want[i] {
+			t.Errorf("word %d = %08x, want %08x", i, got[i], want[i])
+		}
+	}
+}