| Issue |
171709
|
| Summary |
[LLVM 20.1][NVPTX] Jump tables are broken for sm < sm_70
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
jvesely
|
brx instructions are used unconditionally, but require ptx version 6.0+ [1]
This leads to the follwoing failures:
```
$ llc -o - -mcpu sm_50 ~/repos/llvm-project/llvm/test/CodeGen/NVPTX/jump-table.ll | TMPDIR=/tmp ptxas - -c -o /dev/null
ptxas /tmp/tmpxft_000f7701_00000000-0_stdin, line 24; error : Feature '.branchtargets directive' requires PTX ISA .version 6.0 or later
ptxas /tmp/tmpxft_000f7701_00000000-0_stdin, line 29; error : Feature 'brx.idx' requires PTX ISA .version 6.0 or later
ptxas /tmp/tmpxft_000f7701_00000000-0_stdin, line 73; error : Feature '.branchtargets directive' requires PTX ISA .version 6.0 or later
ptxas /tmp/tmpxft_000f7701_00000000-0_stdin, line 80; error : Feature 'brx.idx' requires PTX ISA .version 6.0 or later
ptxas fatal : Ptx assembly aborted due to errors
$
$ llc -o - -mcpu sm_60 ~/repos/llvm-project/llvm/test/CodeGen/NVPTX/jump-table.ll | TMPDIR=/tmp ptxas - -c -o /dev/null
ptxas /tmp/tmpxft_000f6b97_00000000-0_stdin, line 24; error : Feature '.branchtargets directive' requires PTX ISA .version 6.0 or later
ptxas /tmp/tmpxft_000f6b97_00000000-0_stdin, line 29; error : Feature 'brx.idx' requires PTX ISA .version 6.0 or later
ptxas /tmp/tmpxft_000f6b97_00000000-0_stdin, line 73; error : Feature '.branchtargets directive' requires PTX ISA .version 6.0 or later
ptxas /tmp/tmpxft_000f6b97_00000000-0_stdin, line 80; error : Feature 'brx.idx' requires PTX ISA .version 6.0 or later
ptxas fatal : Ptx assembly aborted due to errors
```
Specifying the ptx version through attributes only works for some SMs.
```
$ llc -mattr='ptx60' -o - -mcpu sm_50 jump-table.ll | TMPDIR=/tmp ptxas - -c -o /dev/null
$
```
breaks for others:
```
$ llc -mattr='ptx60' -o - -mcpu sm_60 jump-table.ll | TMPDIR=/tmp ptxas - -c -o /dev/null
ptxas fatal : SM version specified by .target is higher than default SM version assumed
```
[1] https://docs.nvidia.com/cuda/parallel-thread-execution/#control-flow-instructions-brx-idx
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