Re: [PATCH] C inlined assembly for reproducing max<min

["Jose E. Marchesi" <jose.marchesi@oracle.com>]

> On Wed, Nov 22, 2023 at 10:08 AM Yonghong Song <yonghong.song@linux.dev> wrote:
>>
>> > +SEC("?tc")
>> > +__log_level(2)
>> > +int test_verifier_range(void)
>> > +{
>> > +    asm volatile (
>> > +        "r5 = 100; \
>> > +        r5 /= 3; \
>> > +        w5 >>= 7; \
>> > +        r5 &= -386969681; \
>> > +        r5 -= -884670597; \
>> > +        w0 = w5; \
>> > +        if w0 & 0x894b6a55 goto +2; \
>>
>> So actually it is 'if w0 & 0x894b6a55 goto +2' failed
>> the compilation.
>>
>> Indeed, the above operation is not supported in llvm.
>> See
>>    https://github.com/llvm/llvm-project/blob/main/llvm/lib/Target/BPF/BPFInstrFormats.td#L62-L74
>> the missing BPFJumpOp<0x4> which corresponds to JSET.
>>
>> The following llvm patch (on top of llvm-project main branch):
>>
>> diff --git a/llvm/lib/Target/BPF/BPFInstrFormats.td b/llvm/lib/Target/BPF/BPFInstrFormats.td
>> index 841d97efc01c..6ed83d877ac0 100644
>> --- a/llvm/lib/Target/BPF/BPFInstrFormats.td
>> +++ b/llvm/lib/Target/BPF/BPFInstrFormats.td
>> @@ -63,6 +63,7 @@ def BPF_JA   : BPFJumpOp<0x0>;
>>   def BPF_JEQ  : BPFJumpOp<0x1>;
>>   def BPF_JGT  : BPFJumpOp<0x2>;
>>   def BPF_JGE  : BPFJumpOp<0x3>;
>> +def BPF_JSET : BPFJumpOp<0x4>;
>>   def BPF_JNE  : BPFJumpOp<0x5>;
>>   def BPF_JSGT : BPFJumpOp<0x6>;
>>   def BPF_JSGE : BPFJumpOp<0x7>;
>> diff --git a/llvm/lib/Target/BPF/BPFInstrInfo.td b/llvm/lib/Target/BPF/BPFInstrInfo.td
>> index 305cbbd34d27..9e75f35efe70 100644
>> --- a/llvm/lib/Target/BPF/BPFInstrInfo.td
>> +++ b/llvm/lib/Target/BPF/BPFInstrInfo.td
>> @@ -246,6 +246,70 @@ class JMP_RI_32<BPFJumpOp Opc, string OpcodeStr, PatLeaf Cond>
>>     let BPFClass = BPF_JMP32;
>>   }
>>
>> +class JSET_RR<string OpcodeStr>
>> +    : TYPE_ALU_JMP<BPF_JSET.Value, BPF_X.Value,
>> +                   (outs),
>> +                   (ins GPR:$dst, GPR:$src, brtarget:$BrDst),
>> +                   "if $dst "#OpcodeStr#" $src goto $BrDst",
>> +                   []> {
>> +  bits<4> dst;
>> +  bits<4> src;
>> +  bits<16> BrDst;
>> +
>> +  let Inst{55-52} = src;
>> +  let Inst{51-48} = dst;
>> +  let Inst{47-32} = BrDst;
>> +  let BPFClass = BPF_JMP;
>> +}
>> +
>> +class JSET_RI<string OpcodeStr>
>> +    : TYPE_ALU_JMP<BPF_JSET.Value, BPF_K.Value,
>> +                   (outs),
>> +                   (ins GPR:$dst, i64imm:$imm, brtarget:$BrDst),
>> +                   "if $dst "#OpcodeStr#" $imm goto $BrDst",
>> +                   []> {
>> +  bits<4> dst;
>> +  bits<16> BrDst;
>> +  bits<32> imm;
>> +
>> +  let Inst{51-48} = dst;
>> +  let Inst{47-32} = BrDst;
>> +  let Inst{31-0} = imm;
>> +  let BPFClass = BPF_JMP;
>> +}
>> +
>> +class JSET_RR_32<string OpcodeStr>
>> +    : TYPE_ALU_JMP<BPF_JSET.Value, BPF_X.Value,
>> +                   (outs),
>> +                   (ins GPR32:$dst, GPR32:$src, brtarget:$BrDst),
>> +                   "if $dst "#OpcodeStr#" $src goto $BrDst",
>> +                   []> {
>> +  bits<4> dst;
>> +  bits<4> src;
>> +  bits<16> BrDst;
>> +
>> +  let Inst{55-52} = src;
>> +  let Inst{51-48} = dst;
>> +  let Inst{47-32} = BrDst;
>> +  let BPFClass = BPF_JMP32;
>> +}
>> +
>> +class JSET_RI_32<string OpcodeStr>
>> +    : TYPE_ALU_JMP<BPF_JSET.Value, BPF_K.Value,
>> +                   (outs),
>> +                   (ins GPR32:$dst, i32imm:$imm, brtarget:$BrDst),
>> +                   "if $dst "#OpcodeStr#" $imm goto $BrDst",
>> +                   []> {
>> +  bits<4> dst;
>> +  bits<16> BrDst;
>> +  bits<32> imm;
>> +
>> +  let Inst{51-48} = dst;
>> +  let Inst{47-32} = BrDst;
>> +  let Inst{31-0} = imm;
>> +  let BPFClass = BPF_JMP32;
>> +}
>> +
>>   multiclass J<BPFJumpOp Opc, string OpcodeStr, PatLeaf Cond, PatLeaf Cond32> {
>>     def _rr : JMP_RR<Opc, OpcodeStr, Cond>;
>>     def _ri : JMP_RI<Opc, OpcodeStr, Cond>;
>> @@ -265,6 +329,10 @@ defm JULT : J<BPF_JLT, "<", BPF_CC_LTU, BPF_CC_LTU_32>;
>>   defm JULE : J<BPF_JLE, "<=", BPF_CC_LEU, BPF_CC_LEU_32>;
>>   defm JSLT : J<BPF_JSLT, "s<", BPF_CC_LT, BPF_CC_LT_32>;
>>   defm JSLE : J<BPF_JSLE, "s<=", BPF_CC_LE, BPF_CC_LE_32>;
>> +def JSET_RR    : JSET_RR<"&">;
>> +def JSET_RI    : JSET_RI<"&">;
>> +def JSET_RR_32 : JSET_RR_32<"&">;
>> +def JSET_RI_32 : JSET_RI_32<"&">;
>>   }
>>
>>   // ALU instructions
>>
>> can solve your inline asm issue. We will discuss whether llvm compiler
>> should be implementing this instruction from source or not.
>
> I'd say 'yes'. clang/llvm should support such asm syntax.
>
> Jose, Eduard,
> Thoughts?

We already support it in GAS:


  $ echo 'if w0 & 0x894b6a55 goto +2' | bpf-unknown-none-as -mdialect=pseudoc -
  $ bpf-unknown-none-objdump -M hex,pseudoc -d a.out
  
  a.out:     file format elf64-bpfle
  
  
  Disassembly of section .text:
  
  0000000000000000 <.text>:
     0:	46 00 02 00 55 6a 4b 89 	if w0&0x894b6a55 goto 0x2


We weren't aware we were diverging with llvm by doing so.  We support
syntax for all the conditional jump instructions using the following
operators:

  BPF_JEQ    ==
  BPF_JGT    >
  BPF_JSGT   s>
  BPF_JGE    >=
  BPF_JSGE   s>=
  BPF_JLT    <
  BPF_JLST   s<
  BPF_JLE    <=
  BPF_JSLE   s<=
  BPF_JSET   &
  BPF_JNE    !=