[RISCV] def cm.jt as an alias inst of cm.jalt

[VincentWu]

Summary

This use InstAlias to fix the Decoding Conflict between cm.jt and cm.jalt.

The spec of Zc* defines cm.jt and cm.jalt has same encoding, the inst will be decode as cm.jt if $index < 32, it will be decode as cm.jalt in other cases.

In previous implemention, it defined two separate instractions with same encoding. But there is no error when compiling.

but it will report Decoding Conflict when we define cm.mva01s in https://reviews.llvm.org/D132819 .

After testing, I think it may because decoding conflict between cm.jt and cm.jalt. Although it dosen't report error promptly. (I think it might be a issue when generating RISCVGenDisassemblerTables.inc)

[craig.topper]

I'm not sure this is the correct fix. The issue seems to be that we didn't isolate C_FSDSP and ZCmp/ZCmt instructions into separate DecoderNamespaces.

[jrtc27]

Doesn't this point out that the original patch was bogus in allowing you to write cm.jalt 1 despite it really being cm.jt 1, a different instruction with observably different semantics?

[craig.topper]

In D149839#4319700, @jrtc27 wrote:

Doesn't this point out that the original patch was bogus in allowing you to write cm.jalt 1 despite it really being cm.jt 1, a different instruction with observably different semantics?

I'll fix the DecoderNamespace. I might look at fixing the other issue.

[jrtc27]

I'm somewhat surprised TableGen didn't catch that to be honest, though...

[craig.topper]

In D149839#4319714, @jrtc27 wrote:

I'm somewhat surprised TableGen didn't catch that to be honest, though...

Here's the conflict

CM_JALT 101000________10
CM_JT 101000000_____10
CM_MVA01S 101011___11___10
CM_MVSA01 101011___01___10
C_FSDSP 101___________10

As far as tablegen is concerned, the _ parts could be immediates that have custom decoders that only allow values that don't conflict. I think.

[craig.topper]

D149891 adds a DecoderNamespace to Zcmt instructions.

llvm/lib/Target/RISCV/RISCVInstrInfoZc.td

def CM_POP : RVInstZcCPPP<(outs), (ins rlist:$rlist, spimm:$spimm),
 
   let Inst{12-8} = 0b11010;
   let Inst{7-4} = rlist;
   let Inst{3-2} = spimm{5-4};
 }
 }
 
 let Predicates = [HasStdExtZcmt],
-hasSideEffects = 0, mayLoad = 0, mayStore = 0 in {
-def CM_JT : RVInst16CJ<0b101, 0b10, (outs), (ins uimm5:$index),
-                       "cm.jt", "$index">{
-  bits<5> index;
-
-  let Inst{12-7} = 0b000000;
-  let Inst{6-2} = index;
-}
+hasSideEffects = 0, mayLoad = 0, mayStore = 0 in
 
 def CM_JALT : RVInst16CJ<0b101, 0b10, (outs), (ins uimm8:$index),
                          "cm.jalt", "$index">{
   bits<8> index;
 
   let Inst{12-10} = 0b000;
   let Inst{9-2} = index;
 }
-} // Predicates = [HasStdExtZcmt]
 
 
 let Predicates = [HasStdExtZcb, HasStdExtMOrZmmul] in{
 def : CompressPat<(MUL GPRC:$rs1, GPRC:$rs1, GPRC:$rs2),
                   (C_MUL GPRC:$rs1, GPRC:$rs2)>;
 let isCompressOnly = true in
 def : CompressPat<(MUL GPRC:$rs1, GPRC:$rs2, GPRC:$rs1),
                   (C_MUL GPRC:$rs1, GPRC:$rs2)>;

 
 let Predicates = [HasStdExtZcb] in {
 def : InstAlias<"c.lbu $rd, (${rs1})",(C_LBU GPRC:$rd, GPRC:$rs1, 0)>;
 def : InstAlias<"c.lhu $rd, (${rs1})",(C_LHU GPRC:$rd, GPRC:$rs1, 0)>;
 def : InstAlias<"c.lh $rd, (${rs1})", (C_LH GPRC:$rd, GPRC:$rs1, 0)>;
 def : InstAlias<"c.sb $rd, (${rs1})", (C_SB GPRC:$rd, GPRC:$rs1, 0)>;
 def : InstAlias<"c.sh $rd, (${rs1})", (C_SH GPRC:$rd, GPRC:$rs1, 0)>;
 }
+
+let Predicates = [HasStdExtZcmt] in
+def : InstAlias<"cm.jt $index",(CM_JALT uimm5:$index)>;

llvm/test/MC/RISCV/rv32zcmt-valid.s

 # RUN: llvm-mc %s -triple=riscv32 -mattr=+experimental-zcmt\
 # RUN:  -riscv-no-aliases -show-encoding \
 # RUN:     | FileCheck -check-prefixes=CHECK-ASM,CHECK-ASM-AND-OBJ %s
 # RUN: llvm-mc -filetype=obj -triple=riscv32 -mattr=+experimental-zcmt\
 # RUN:  -mattr=m < %s \
 # RUN:     | llvm-objdump --mattr=+experimental-zcmt\
 # RUN:  -M no-aliases -d -r - \
-# RUN:     | FileCheck --check-prefixes=CHECK-OBJ,CHECK-ASM-AND-OBJ %s
+# RUN:     | FileCheck --check-prefixes=CHECK-ASM-AND-OBJ %s
 # RUN: llvm-mc %s -triple=riscv64 -mattr=+experimental-zcmt\
 # RUN:  -riscv-no-aliases -show-encoding \
 # RUN:     | FileCheck -check-prefixes=CHECK-ASM,CHECK-ASM-AND-OBJ %s
 # RUN: llvm-mc -filetype=obj -triple=riscv64 -mattr=+experimental-zcmt\
 # RUN:  -mattr=m < %s \
 # RUN:     | llvm-objdump --mattr=+experimental-zcmt\
 # RUN:  -M no-aliases -d -r - \
-# RUN:     | FileCheck --check-prefixes=CHECK-OBJ,CHECK-ASM-AND-OBJ %s
+# RUN:     | FileCheck --check-prefixes=CHECK-ASM-AND-OBJ %s
 #
 # RUN: not llvm-mc -triple riscv32 \
 # RUN:     -riscv-no-aliases -show-encoding < %s 2>&1 \
 # RUN:     | FileCheck -check-prefixes=CHECK-NO-EXT %s
 # RUN: not llvm-mc -triple riscv64 \
 # RUN:     -riscv-no-aliases -show-encoding < %s 2>&1 \
 # RUN:     | FileCheck -check-prefixes=CHECK-NO-EXT %s
 
-# CHECK-ASM-AND-OBJ: cm.jt 1
+# CHECK-ASM-AND-OBJ: cm.jalt 1
 # CHECK-ASM: encoding: [0x06,0xa0]
 # CHECK-NO-EXT: error: instruction requires the following: 'Zcmt' (table jump instuctions for code-size reduction){{$}}
 cm.jt 1
 
-# CHECK-ASM: cm.jalt 1
-# CHECK-OBJ: cm.jt 1
+# CHECK-ASM-AND-OBJ: cm.jalt 1
 # CHECK-ASM: encoding: [0x06,0xa0]
 # CHECK-NO-EXT: error: instruction requires the following: 'Zcmt' (table jump instuctions for code-size reduction){{$}}
 cm.jalt 1
 
 # CHECK-ASM-AND-OBJ: cm.jalt 32
 # CHECK-ASM: encoding: [0x82,0xa0]
 # CHECK-NO-EXT: error: instruction requires the following: 'Zcmt' (table jump instuctions for code-size reduction){{$}}
 cm.jalt 32