Issues in Vector Add Instruction Machine Code Emission

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1

Hello,
I am trying to emit binary for my implemented vector instructions. Although yet i havent done any change or addition in MC framework, For vector load instruction there are no error coming. But for vector add

instruction is something like this;

%R_0_REG2048b_1 = P_256B_VADD %R_0_REG2048b_1, %R_0_REG2048b_0

I am getting the following error:

Unknown immediate size
UNREACHABLE executed at /lib/Target/X86/MCTargetDesc/X86BaseInfo.h:574!

i made extensive use of gdb and after debugging i found the line with issue in X86MCCodeEmitter.cpp.

Here NumOps=3 (all registers). and CurOp is 1st initialized to 0.

then, the following code gets executed;

case X86II::MRMDestReg: {
EmitByte(BaseOpcode, CurByte, OS);
unsigned SrcRegNum = CurOp + 1; //SrcRegNum=1

EmitRegModRMByte(MI.getOperand(CurOp),
GetX86RegNum(MI.getOperand(SrcRegNum)), CurByte, OS);
CurOp = SrcRegNum + 1;
break;
}

so here CurOp becomes 2.

After this;

it comes to;

else {
// If there is a remaining operand, it must be a trailing immediate. Emit it
// according to the right size for the instruction. Some instructions
// (SSE4a extrq and insertq) have two trailing immediates.
while (CurOp != NumOps && NumOps - CurOp <= 2) {
EmitImmediate(MI.getOperand(CurOp++), MI.getLoc(),
X86II::getSizeOfImm(TSFlags), getImmFixupKind(TSFlags),
CurByte, OS, Fixups);
}

here CurOp=2 !=NumOps=3 && 3-2<=2
so while condition is satisfied and it goes to emitimmediate which is wrong and there prints error message.

Since, there are no immediate involved in instruction, it should not go to emitimmediate. How to solve this issue?

Please help.

Thank You

2

Does your ADD instruction have VEX_4V or EVEX_4V as part of its declaration in the td file?

3

Thank You.

My add instruction has TA as follows:

def P_256B_VADD : I<0xE1, MRMDestReg, (outs VRP_2048:$dst), (ins VRP_2048:$src1, VRPIM_2048:$src2),“P_256B_VADD\t{$src1, $src2, $dst|$dst, $src1, $src2}”, [(set VRP_2048:$dst, (add (v64i32 VRP_2048:$src1), (v64i32 VRP_2048:$src2)))]>, TA;

so i defined;

bool HasTA = TSFlags & X86II::TA; in x86MCCodeEmitter.cpp

then used this condition;

if(HasTA)
++SrcRegNum;

now getting no error.

please tell me whether my method is correct? Also please confirm this whether i need to make changes in MC framework to emit binary code of my vector instructions. So far i made no changes or additions in MC framework or any of the file (except the above discussed) and still getting the correct machine code.

Is it right way?

4

That is not correct. You should add VEX_4V. TA tells the X86 encoder that the instruction opcode belongs on the 3 byte opcode 0x0F 0x3A page in the Intel manual.

5

You are right. But when i defined my instruction as follows:
def P_256B_VADD : I<0xE1, MRMDestReg, (outs VRP_2048:$dst), (ins VRP_2048:$src1, VRPIM_2048:$src2),“P_256B_VADD\t{$src1, $src2, $dst|$dst, $src1, $src2}”, [(set VRP_2048:$dst, (add (v64i32 VRP_2048:$src1), (v64i32 VRP_2048:$src2)))]>, VEX_4V;

I get opcode conflicts? Then what to do?

6

Leave TA. Put both.

7

Sorry to ask but what does it mean to put both?

8

VEX_4V tells the encoder to use the VEX.vvvv field to encode one of the operands. Without it the encoder assumes that the destination and one of the sources must be the same physical register.

TA indicates which of the opcode maps the instruction belongs to. This corresponds to encoding 0x3 of the VEX.mmmmm field.

9

Thank You.

I used EVEX_4V with all the instructions. I replaced TA and EVEX both with EVEX_4V. Now, I am getting following error:

llvm-tblgen: /utils/TableGen/X86RecognizableInstr.cpp:687: void llvm::X86Disassembler::RecognizableInstr::emitInstructionSpecifier(): Assertion `numPhysicalOperands >= 2 + additionalOperands && numPhysicalOperands <= 4 + additionalOperands && “Unexpected number of operands for MRMSrcMemFrm”’ failed

What to do now?

10

Not all instructions can use EVEX_4V. Move instructions in particular cannot because they don’t have 2 sources.

What do you intend to do with the binary output once you have it? You don’t seem to be targeting a particular binary definition so its effectively just random numbers.

11

Thank You,
I changed TA to EVEX or EVEX_4V. But now i am getting following error:

Invalid prefix!
UNREACHABLE executed at /lib/Target/X86/MCTargetDesc/X86MCCodeEmitter.cpp:647!

12

Put the TA’s back. EVEX/EVEX_4V does not replace TA. They are for different things. An EVEX/EVEX_4V instruction must use one of T8, TA, XOP8, XOP9, XOPA.

13

I was getting same error when i keep both EVEX/EVEX_4V and TA. So, i restored my original instructions and for that i have to include

bool HasTA = TSFlags & X86II::TA; in x86MCCodeEmitter.cpp

then used this condition;

if(HasTA)
++SrcRegNum;

in order to emit binary correctly.

Is it right?

14

No, its not right. What error were you getting with EVEX/EVEX_4V and TA?

15

the output of backtrace is follows:

(gdb) bt
#0 0x00007ffff6b79267 in __GI_raise (sig=sig@entry=6)
at …/sysdeps/unix/sysv/linux/raise.c:55
#1 0x00007ffff6b7aeca in __GI_abort () at abort.c:89
#2 0x0000000001960700 in llvm::llvm_unreachable_internal (
msg=0x2224614 “Invalid op prefix!”,
file=0x222411f “/lib/Target/X86/MCTargetDesc/X86MCCodeEmitter.cpp”, line=681)
at /lib/Support/ErrorHandling.cpp:118
#3 0x0000000000c05c57 in (anonymous namespace)::X86MCCodeEmitter::EmitVEXOpcodePrefix (this=0x3037d00, TSFlags=1126109823418401, CurByte=@0x7fffffffae7c: 0,
MemOperand=1, MI=…, Desc=…, OS=…)
at /lib/Target/X86/MCTargetDesc/X86MCCodeEmitter.cpp:681
#4 0x0000000000c03add in (anonymous namespace)::X86MCCodeEmitter::encodeInstruction (this=0x3037d00, MI=…, OS=…, Fixups=…, STI=…)
at /lib/Target/X86/MCTargetDesc/X86MCCodeEmitter.cpp:1227
#5 0x00000000015894f3 in llvm::MCELFStreamer::EmitInstToData (this=0x304db90,
Inst=…, STI=…)
at /lib/MC/MCELFStreamer.cpp:478
#6 0x00000000015abe00 in llvm::MCObjectStreamer::EmitInstruction (
this=0x304db90, Inst=…, STI=…)
at /lib/MC/MCObjectStreamer.cpp:244
#7 0x00000000007a04ab in llvm::X86AsmPrinter::EmitAndCountInstruction (this=
0x3052950, Inst=…)
at /lib/Target/X86/X86MCInstLower.cpp:105
#8 0x00000000007a341c in llvm::X86AsmPrinter::EmitInstruction (
this=0x3052950, MI=0x30a1e10)
at /lib/Target/X86/X86MCInstLower.cpp:1737
#9 0x0000000000dde5d6 in llvm::AsmPrinter::EmitFunctionBody (this=0x3052950)
—Type to continue, or q to quit—
at /lib/CodeGen/AsmPrinter/AsmPrinter.cpp:939
#10 0x0000000000796fc1 in llvm::X86AsmPrinter::runOnMachineFunction (
this=0x3052950, MF=…)
at /lib/Target/X86/X86AsmPrinter.cpp:70
#11 0x000000000104d7f1 in llvm::MachineFunctionPass::runOnFunction (
this=0x3052950, F=…)
at /lib/CodeGen/MachineFunctionPass.cpp:62
#12 0x00000000014776ff in llvm::FPPassManager::runOnFunction (this=0x302f5d0,
F=…) at /lib/IR/LegacyPassManager.cpp:1513
#13 0x0000000001477a15 in llvm::FPPassManager::runOnModule (this=0x302f5d0,
M=…) at /lib/IR/LegacyPassManager.cpp:1534
#14 0x00000000014781aa in (anonymous namespace)::MPPassManager::runOnModule (
this=0x3022d30, M=…)
at /lib/IR/LegacyPassManager.cpp:1590
#15 0x0000000001477cd6 in llvm::legacy::PassManagerImpl::run (this=0x301d250,

M=…) at /lib/IR/LegacyPassManager.cpp:1693
#16 0x00000000014786c1 in llvm::legacy::PassManager::run (this=0x7fffffffd778,
M=…) at /lib/IR/LegacyPassManager.cpp:1724
#17 0x000000000076bd1e in compileModule (argv=0x7fffffffdf68, Context=…)
at /tools/llc/llc.cpp:528
#18 0x000000000076a209 in main (argc=5, argv=0x7fffffffdf68)
at /tools/llc/llc.cpp:285
(gdb)
(gdb)

16

That’s a different error. The error you had before said “invalid prefix!” and was on line 647. This one says “Invalid op prefix!” and is on line 681.

You need to add one of PS, PD, XS, XD to your instructions as well.

switch (TSFlags & X86II::OpPrefixMask) {
default: llvm_unreachable(“Invalid op prefix!”);
case X86II::PS: VEX_PP = 0x0; break; // none
case X86II::PD: VEX_PP = 0x1; break; // 66
case X86II::XS: VEX_PP = 0x2; break; // F3
case X86II::XD: VEX_PP = 0x3; break; // F2
}