cmd/asm: rename R18 to R18_PLATFORM on ARM64

In ARM64 ABI, R18 is the "platform register", the use of which is
OS specific. The OS could choose to reserve this register. In
practice, it seems fine to use R18 on Linux but not on darwin (iOS).

Rename R18 to R18_PLATFORM to prevent accidental use. There is no
R18 usage within the standard library (besides tests, which are
updated).

Fixes #26110

Change-Id: Icef7b9549e2049db1df307a0180a3c90a12d7a84
Reviewed-on: https://go-review.googlesource.com/c/147218
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/asm.html

@@ -740,6 +740,7 @@ The ARM64 port is in an experimental state.
 
 <p>
 <code>R18</code> is the "platform register", reserved on the Apple platform.
+To prevent accidental misuse, the register is named <code>R18_PLATFORM</code>.
 <code>R27</code> and <code>R28</code> are reserved by the compiler and linker.
 <code>R29</code> is the frame pointer.
 <code>R30</code> is the link register.

src/cmd/asm/internal/arch/arch.go

@@ -258,6 +258,9 @@ func archArm64() *Arch {
 	for i := arm64.REG_R0; i <= arm64.REG_R31; i++ {
 		register[obj.Rconv(i)] = int16(i)
 	}
+	// Rename R18 to R18_PLATFORM to avoid accidental use.
+	register["R18_PLATFORM"] = register["R18"]
+	delete(register, "R18")
 	for i := arm64.REG_F0; i <= arm64.REG_F31; i++ {
 		register[obj.Rconv(i)] = int16(i)
 	}

src/cmd/asm/internal/asm/operand_test.go

@@ -607,6 +607,7 @@ var arm64OperandTests = []operandTest{
 	{"R0", "R0"},
 	{"R10", "R10"},
 	{"R11", "R11"},
+	{"R18_PLATFORM", "R18"},
 	{"$4503601774854144.0", "$(4503601774854144.0)"},
 	{"$runtime·badsystemstack(SB)", "$runtime.badsystemstack(SB)"},
 	{"ZR", "ZR"},

src/cmd/asm/internal/asm/testdata/arm64.s

@@ -47,8 +47,8 @@ TEXT	foo(SB), DUPOK|NOSPLIT, $-8
 	ADD	R2.SXTX<<1, RSP, RSP            // ffe7228b
 	ADD	ZR.SXTX<<1, R2, R3              // 43e43f8b
 	ADDW	R2.SXTW, R10, R12               // 4cc1220b
-	ADD	R18.UXTX, R14, R17              // d161328b
-	ADDSW	R18.UXTW, R14, R17              // d141322b
+	ADD	R19.UXTX, R14, R17              // d161338b
+	ADDSW	R19.UXTW, R14, R17              // d141332b
 	ADDS	R12.SXTX, R3, R1                // 61e02cab
 	SUB	R19.UXTH<<4, R2, R21            // 553033cb
 	SUBW	R1.UXTX<<1, R3, R2              // 6264214b
@@ -144,7 +144,7 @@ TEXT	foo(SB), DUPOK|NOSPLIT, $-8
 	MOVD	(R2)(R6.SXTW), R4               // 44c866f8
 	MOVD	(R3)(R6), R5                    // MOVD	(R3)(R6*1), R5                  // 656866f8
 	MOVD	(R2)(R6), R4                    // MOVD	(R2)(R6*1), R4                  // 446866f8
-	MOVWU	(R19)(R18<<2), R18              // 727a72b8
+	MOVWU	(R19)(R20<<2), R20              // 747a74b8
 	MOVD	(R2)(R6<<3), R4                 // 447866f8
 	MOVD	(R3)(R7.SXTX<<3), R8            // 68f867f8
 	MOVWU	(R5)(R4.UXTW), R10              // aa4864b8
@@ -154,7 +154,7 @@ TEXT	foo(SB), DUPOK|NOSPLIT, $-8
 	MOVHU	(R1)(R2<<1), R5                 // 25786278
 	MOVB	(R9)(R3.UXTW), R6               // 2649a338
 	MOVB	(R10)(R6), R15                  // MOVB	(R10)(R6*1), R15                // 4f69a638
-	MOVH	(R5)(R7.SXTX<<1), R18           // b2f8a778
+	MOVH	(R5)(R7.SXTX<<1), R19           // b3f8a778
 	MOVH	(R8)(R4<<1), R10                // 0a79a478
 	MOVW	(R9)(R8.SXTW<<2), R19           // 33d9a8b8
 	MOVW	(R1)(R4.SXTX), R11              // 2be8a4b8

src/cmd/internal/obj/arm64/doc.go

@@ -89,7 +89,7 @@ such as str, stur, strb, sturb, strh, sturh stlr, stlrb. stlrh, st1.
   Examples:
     MOVD R29, 384(R19)    <=>    str x29, [x19,#384]
     MOVB.P R30, 30(R4)    <=>    strb w30, [x4],#30
-    STLRH R21, (R18)      <=>    stlrh w21, [x18]
+    STLRH R21, (R19)      <=>    stlrh w21, [x19]
 
 (2) MADD, MADDW, MSUB, MSUBW, SMADDL, SMSUBL, UMADDL, UMSUBL <Rm>, <Ra>, <Rn>, <Rd>
 
@@ -127,7 +127,7 @@ such as str, stur, strb, sturb, strh, sturh stlr, stlrb. stlrh, st1.
 
   Examples:
     CCMN VS, R13, R22, $10     <=>    ccmn x13, x22, #0xa, vs
-    CCMPW HS, R18, R14, $11    <=>    ccmp w18, w14, #0xb, cs
+    CCMPW HS, R19, R14, $11    <=>    ccmp w19, w14, #0xb, cs
 
 (9) CSEL, CSELW, CSNEG, CSNEGW, CSINC, CSINCW <cond>, <Rn>, <Rm>, <Rd> ;
 FCSELD, FCSELS <cond>, <Fn>, <Fm>, <Fd>
@@ -144,12 +144,12 @@ FCSELD, FCSELS <cond>, <Fn>, <Fm>, <Fd>
 
   Examples:
     STLXR ZR, (R15), R16    <=>    stlxr w16, xzr, [x15]
-    STXRB R9, (R21), R18    <=>    stxrb w18, w9, [x21]
+    STXRB R9, (R21), R19    <=>    stxrb w19, w9, [x21]
 
 (12) STLXP, STLXPW, STXP, STXPW (<Rf1>, <Rf2>), (<Rn|RSP>), <Rs>
 
   Examples:
-    STLXP (R17, R18), (R4), R5      <=>    stlxp w5, x17, x18, [x4]
+    STLXP (R17, R19), (R4), R5      <=>    stlxp w5, x17, x19, [x4]
     STXPW (R30, R25), (R22), R13    <=>    stxp w13, w30, w25, [x22]
 
 2. Expressions for special arguments.
@@ -173,7 +173,7 @@ Extended registers are written as <Rm>{.<extend>{<<<amount>}}.
 <extend> can be UXTB, UXTH, UXTW, UXTX, SXTB, SXTH, SXTW or SXTX.
 
   Examples:
-    ADDS R18.UXTB<<4, R9, R26     <=>    adds x26, x9, w18, uxtb #4
+    ADDS R19.UXTB<<4, R9, R26     <=>    adds x26, x9, w19, uxtb #4
     ADDSW R14.SXTX, R14, R6       <=>    adds w6, w14, w14, sxtx
 
 Memory references: [<Xn|SP>{,#0}] is written as (Rn|RSP), a base register and an immediate