<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/133779>133779</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [RISCV] Introduction of Zcb instructions in lr/sc sequences may not meet forward progress guarantee
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            backend:RISC-V
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          asb
      </td>
    </tr>
</table>

<pre>
    I've written up a fuller description here https://github.com/riscv/riscv-isa-manual/issues/1938 in a request to confirm/clarify the current wording. Zcb introduced some new compressed instructions that map from 'I' instructions (c.zext.b and c.not). RISCVExpandAtomicPass takes great care in only producing instructions within the lr/sc sequence that confirm with the forward progess guarantee, but for at least the atomicrmw nand expensions, we produce a `xori rd, rs, -1` (for which `not rd, rs` is a pseudoinstruction), and the final stage compression later converts this to c.not. A plausible reading of the current wording on the forward progress guarantee suggests that the presence of `c.not` breaks it. Or alternatively, it's just that the text hasn't been updated since Zcb was merged and that was never the intent.

Filing an issue to keep track of this in case we need to make adjustments, depending on the outcome of https://github.com/riscv/riscv-isa-manual/issues/1938

```
$ cat t.c
char c;

void foo(void) {
  __sync_fetch_and_nand (&c, 1);
}
$ clang --target=riscv64-linux-gnu -march=rv64gc_zcb -O3 t.c -c -o t.clang.o
$ llvm-objdump -d -Mno-aliases t.clang.o

t.clang.o:      file format elf64-littleriscv

Disassembly of section .text:

0000000000000000 <foo>:
       0: 00000517      auipc   a0, 0x0
       4: 00050513      addi    a0, a0, 0x0
       8: ffc57593      andi    a1, a0, -0x4
       c: 890d          c.andi  a0, 0x3
       e: 050e          c.slli  a0, 0x3
 10: 0ff00613           addi    a2, zero, 0xff
      14: 00a6163b      sllw    a2, a2, a0
      18: 4685          c.li    a3, 0x1
      1a: 00a6953b      sllw    a0, a3, a0
      1e: 1605a6af      lr.w.aqrl       a3, (a1)
      22: 00a6f733      and     a4, a3, a0
      26: 9f75          c.not   a4
      28: 8f35          c.xor   a4, a3
      2a: 8f71          c.and   a4, a2
 2c: 8f35               c.xor   a4, a3
      2e: 1ae5a72f      sc.w.rl a4, a4, (a1)
      32: e301          c.bnez  a4, 0x32 <foo+0x32>
      34: 8082  c.jr    ra
```
</pre>
<img width="1" height="1" alt="" src="http://email.email.llvm.org/o/eJysVl2P4joS_TXmpZTIsRNCHnhgpm9L87C6q13pPuxLq-JUwNOOzbUdoOfXr-zw0d0zGmmlRYgAOaeqzqmyYwxB7y3RljVfWPO0wjkenN9i6Fe9G96235hoTwRnr2MkC_MREMbZGPIwUFBeH6N2Fg7kCQ4xHgOTOyaemXje63iY-1K5iYlnr4M63a6FDlhMaGc0TDzrEGYKTDxXndyAtoDg6e-ZQoToQDk7ap9CKINej28QDwRq9p5shLPzg7b7Ev6jetA2ejfMigYIbiKwdAblpqOnEGgAbUP0s0rlBogHjDDhEUbvJmCiTTo_QpjYqPIHXWLZA9oBVGldZKIr4V_f_v31rz8uR7TDLrpJq39iCBDxlQLsPWEEhZ6SFGfNGxxzVdruP8Y_63jQNssxnonnoCAk3VbRUt5VegZm2Oj8Gf2QAu4pBNjP6NFGIia-Qj_HBACMYAiTeQcCzOX56Qw2SaDLkWxI2RPjTNfSCBDYml-c1-CHdMtnQFGxNU8-pLjng1aHBLMuPlBrDjoAwjHQPLh3-pjoEiRlzaVriwZCxD3de5LmxmAkn5SeyMfUFh1y15PXJezgaHAOujcEnjC1Gtz4qwkAZ3-yyH_wCMK831OI194ncKoi2-3GJGxp8JpD7wlfA-hYwp8e0ETyFqM-kXlLmnRkog3wfc4mX2NFukQ4YLBMtBF6ymtlwJiGUaccaULPGGAiv6fhagzG_J-lE_kcRttINpaM7xjfPWuTpKGFvEaSMa9ER4ge1etihA5pzBQGSu20RENCTfhKgEOqcCIbczMHOpJ9b5Wbo0rLxI3_j4W7VMzW_PrmOyZqUMmdUjG-Uwf0oJj8sgBPTg8wOsfEJn1logPWpnsALy_hzaqXkaI6vKAdXvLoMrFhYq2SkirN1hKofbolMmj3UBQR_Z4ik0-53nVdGG3nS7G3MxQTenVIt07req9efqgeij9lqg8KBYVL31KY0l2DGnOaCtd_H-bpCMUAxT-sK9BoDBQ-gvnu8VPuGO9GbfIwThiBzJgLidHQYmdmPOmAIdDUm7fUg0B53UCZJikHSW_-6QVMfk22yT8WCCwvzuQOMqCp2vwP4x3O-qjSlSfX-IU_8PUV3_Cmkhk7DPoO_QVhkwjjqJq26eQ9gV1I1YNU8Ev9YKnE2nR8gNuL8U6VN941jXwQKNfVcPpICMb8RKgWzePI-bp6lHTVIRLyB3m3MMbxnqO6asd1tZb9jReMOd9518-H_CrLr9eb5mNdS1VyyVE94HhL0TWyfx9-sVd-Dp9lV2ve4BrHW3jjy3OJf3tzz8HEBvPw34hC3BKNrXzflvRZ_zKXWCdKN7afpKStL5PuwKx5M8pPwIvz76Pf4bjA2-qnXj_gIsGF-t8CL-YgNdiKuzlBledysWYh1L80SGaDSPJPVfWWfty5_CLFbV2JL-lXWl73EHleNnwjFur3VKXH95vdatjKoZMdrmhbtbWs61aKanXYDnWDshm7TVVT223U0JOsh7ZpG6JBclzpreCi4VJW1aZpRFXyVmFdU6ek6Ma6WrOa04TalGkvKp3fr_Kuu62kbNtuZbAnE_LZTYge1SvZgcldOqAUfzEh0onOb_M-1s_7wGpudIjhES3qaPLZL59pWPME366nqLwZufF6snp3btH2p_NKgAnfIB0MJqL4m4fwavZm-5unTarreimO3n0nFT88aBbVp634bwAAAP__6Lk0_g">