[MIPS] Introduce NAL instruction support for Mipsr6 and prer6 (#84429)

NAL is an assembly idiom on Pre-R6 instruction sets (which is
implemented in binutils), or an actual instruction on Release 6
instruction set, and is used to read the PC, due to the nature of the
MIPS architecture.

Since we can't read the PC directly, on pre-R6 we use a always-not-taken
Branch and Link operation to the address of the next instruction, which
effectively writes the address to $31, thus PC is read with offset +8.

MIPS Release 6 removed the conventional Branch and Link instructions,
but kept NAL as an actual instruction for compatibility on the assembly
level. The instruction has the same encoding of the pre-R6 ones, and
with the same behavior: PC + 8 -> $31.

Author: anbbna

llvm/lib/Target/Mips/Mips32r6InstrFormats.td

@@ -86,6 +86,7 @@ def OPCODE5_BC1NEZ : OPCODE5<0b01101>;
 def OPCODE5_BC2EQZ : OPCODE5<0b01001>;
 def OPCODE5_BC2NEZ : OPCODE5<0b01101>;
 def OPCODE5_BGEZAL : OPCODE5<0b10001>;
+def OPCODE5_NAL : OPCODE5<0b10000>;
 def OPCODE5_SIGRIE : OPCODE5<0b10111>;
 // The next four constants are unnamed in the spec. These names are taken from
 // the OPGROUP names they are used with.
@@ -201,6 +202,16 @@ class BAL_FM : MipsR6Inst {
   let Inst{15-0} = offset;
 }
 
+// NAL for Release 6
+class NAL_FM : MipsR6Inst {
+  bits<32> Inst;
+
+  let Inst{31-26} = OPGROUP_REGIMM.Value;
+  let Inst{25-21} = 0b00000; 
+  let Inst{20-16} = OPCODE5_NAL.Value;
+  let Inst{15-0} = 0x00;
+}
+
 class COP0_EVP_DVP_FM<bits<1> sc> : MipsR6Inst {
   bits<5> rt;
 

llvm/lib/Target/Mips/Mips32r6InstrInfo.td

@@ -73,6 +73,7 @@ class AUI_ENC    : AUI_FM;
 class AUIPC_ENC  : PCREL16_FM<OPCODE5_AUIPC>;
 
 class BAL_ENC   : BAL_FM;
+class NAL_ENC   : NAL_FM;
 class BALC_ENC  : BRANCH_OFF26_FM<0b111010>;
 class BC_ENC    : BRANCH_OFF26_FM<0b110010>;
 class BEQC_ENC  : CMP_BRANCH_2R_OFF16_FM<OPGROUP_ADDI>,
@@ -381,6 +382,12 @@ class BC_DESC_BASE<string instr_asm, DAGOperand opnd> : BRANCH_DESC_BASE,
   bit isCTI = 1;
 }
 
+class NAL_DESC_BASE<string instr_asm> : BRANCH_DESC_BASE,
+    MipsR6Arch<instr_asm> {
+  string AsmString = instr_asm;
+  bit isCTI = 1;
+}
+
 class CMP_BC_DESC_BASE<string instr_asm, DAGOperand opnd,
                        RegisterOperand GPROpnd> : BRANCH_DESC_BASE,
                                                   MipsR6Arch<instr_asm> {
@@ -424,6 +431,12 @@ class BAL_DESC : BC_DESC_BASE<"bal", brtarget> {
   bit isCTI = 1;
 }
 
+class NAL_DESC : NAL_DESC_BASE<"nal"> {
+  bit hasDelaySlot = 1;
+  list<Register> Defs = [RA];
+  bit isCTI = 1;
+}
+
 class BALC_DESC : BC_DESC_BASE<"balc", brtarget26> {
   bit isCall = 1;
   list<Register> Defs = [RA];
@@ -868,6 +881,8 @@ def AUI : R6MMR6Rel, AUI_ENC, AUI_DESC, ISA_MIPS32R6;
 def AUIPC : R6MMR6Rel, AUIPC_ENC, AUIPC_DESC, ISA_MIPS32R6;
 def BAL : BAL_ENC, BAL_DESC, ISA_MIPS32R6;
 def BALC : R6MMR6Rel, BALC_ENC, BALC_DESC, ISA_MIPS32R6;
+def NAL : NAL_ENC, NAL_DESC, ISA_MIPS32R6;
+
 let AdditionalPredicates = [NotInMicroMips] in {
   def BC1EQZ : BC1EQZ_ENC, BC1EQZ_DESC, ISA_MIPS32R6, HARDFLOAT;
   def BC1NEZ : BC1NEZ_ENC, BC1NEZ_DESC, ISA_MIPS32R6, HARDFLOAT;
@@ -948,7 +963,6 @@ let AdditionalPredicates = [NotInMicroMips] in {
   def MUL_R6 : R6MMR6Rel, MUL_R6_ENC, MUL_R6_DESC, ISA_MIPS32R6;
   def MULU   : R6MMR6Rel, MULU_ENC, MULU_DESC, ISA_MIPS32R6;
 }
-def NAL; // BAL with rd=0
 let AdditionalPredicates = [NotInMicroMips] in {
   def PREF_R6 : R6MMR6Rel, PREF_ENC, PREF_DESC, ISA_MIPS32R6;
   def RINT_D : RINT_D_ENC, RINT_D_DESC, ISA_MIPS32R6, HARDFLOAT;

llvm/lib/Target/Mips/MipsInstrInfo.td

@@ -3049,6 +3049,9 @@ def : MipsInstAlias<"divu $rd, $imm", (UDivIMacro GPR32Opnd:$rd, GPR32Opnd:$rd,
                                                   simm32:$imm), 0>,
       ISA_MIPS1_NOT_32R6_64R6;
 
+
+def : MipsInstAlias<"nal", (BLTZAL ZERO, 0), 1>, ISA_MIPS1_NOT_32R6_64R6;
+
 def SRemMacro : MipsAsmPseudoInst<(outs GPR32Opnd:$rd),
                                   (ins GPR32Opnd:$rs, GPR32Opnd:$rt),
                                   "rem\t$rd, $rs, $rt">,

llvm/lib/Target/Mips/MipsScheduleGeneric.td

@@ -285,7 +285,7 @@ def GenericWriteJumpAndLink : SchedWriteRes<[GenericIssueCTISTD]> {
 // jalr, jr.hb, jr, jalr.hb, jarlc, jialc
 def : InstRW<[GenericWriteJump], (instrs B, BAL, BAL_BR, BEQ, BNE, BGTZ, BGEZ,
                                   BLEZ, BLTZ, BLTZAL, J, JALX, JR, JR_HB, ERET,
-                                  ERet, ERETNC, DERET)>;
+                                  ERet, ERETNC, DERET, NAL)>;
 
 def : InstRW<[GenericWriteJump], (instrs BEQL, BNEL, BGEZL, BGTZL, BLEZL,
                                   BLTZL)>;

llvm/test/MC/Mips/mips32/nal.s

@@ -0,0 +1,14 @@
+# RUN: llvm-mc %s -triple=mipsel-linux-gnu -filetype=obj -o - | \
+# RUN:		 llvm-objdump --no-print-imm-hex -d - | FileCheck %s --check-prefix=MIPS32-EL
+# RUN: llvm-mc %s -triple=mips-linux-gnu -filetype=obj -o - | \
+# RUN:		 llvm-objdump --no-print-imm-hex -d - | FileCheck %s --check-prefix=MIPS32-EB
+
+# Whether it is a macro or an actual instruction, it always has a delay slot.
+# Ensure the delay slot is filled correctly.
+# MIPS32-EL:		00 00 10 04   bltzal  $zero, 0x4
+# MIPS32-EL-NEXT: 	00 00 00 00   nop
+# MIPS32-EB:		04 10 00 00   bltzal  $zero, 0x4
+# MIPS32-EB-NEXT:	00 00 00 00   nop
+
+nal_test:
+	nal

llvm/test/MC/Mips/mips32r6/nal.s

@@ -0,0 +1,21 @@
+# RUN: llvm-mc %s -triple=mipsisa32r6el-linux-gnu -filetype=obj -o - | \
+# RUN:		 llvm-objdump --no-print-imm-hex -d - | FileCheck %s --check-prefix=MIPS32R6-EL
+# RUN: llvm-mc %s -triple=mipsisa32r6-linux-gnu -filetype=obj -o - | \
+# RUN: 		 llvm-objdump --no-print-imm-hex -d - | FileCheck %s --check-prefix=MIPS32R6-EB
+
+# Whether it is a macro or an actual instruction, it always has a delay slot.
+# Ensure the delay slot is filled correctly.
+# Also ensure that NAL does not reside in a forbidden slot.
+# MIPS32R6-EL:		00 00 80 f8   bnezc	$4, 0x4
+# MIPS32R6-EL-NEXT:	00 00 00 00   nop
+# MIPS32R6-EL:		00 00 10 04   nal
+# MIPS32R6-EL-NEXT:	00 00 00 00   nop
+# MIPS32R6-EB:		f8 80 00 00   bnezc	$4, 0x4
+# MIPS32R6-EB-NEXT:	00 00 00 00   nop
+# MIPS32R6-EB:		04 10 00 00   nal
+# MIPS32R6-EB-NEXT:	00 00 00 00   nop
+
+nal_test:
+	# We generate a fobidden solt just for testing.
+	bnezc $a0, 0
+	nal