cuda clang: Fix argument order for __reduce_max_sync

[AustinSchuh]

The following cuda kernel would crash with an "an illegal instruction was encountered" message.

__global__ void testcode(const float* data, unsigned *max_value) {
    unsigned r = static_cast<unsigned>(data[threadIdx.x]);

    const unsigned mask = __ballot_sync(0xFFFFFFFF, true);

    unsigned mx = __reduce_max_sync(mask, r);
    atomicMax(max_value, mx);
}

Digging into the ptx from both nvcc and clang, I discovered that the arguments for the mask and value were swapped. This swaps them back.

Fixes: #131415

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[llvmbot]

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@llvm/pr-subscribers-backend-nvptx

Author: Austin Schuh (AustinSchuh)

Changes

The following cuda kernel would crash with an "an illegal instruction was encountered" message.

global void testcode(const float* data, unsigned *max_value) {
unsigned r = static_cast<unsigned>(data[threadIdx.x]);

const unsigned mask = __ballot_sync(0xFFFFFFFF, true);

unsigned mx = __reduce_max_sync(mask, r);
atomicMax(max_value, mx);

}

Digging into the ptx from both nvcc and clang, I discovered that the arguments for the mask and value were swapped. This swaps them back.

Fixes: #131415

---

Full diff: https://github.com/llvm/llvm-project/pull/132881.diff

1 Files Affected:

• (modified) llvm/lib/Target/NVPTX/NVPTXIntrinsics.td (+1-1)

diff --git a/llvm/lib/Target/NVPTX/NVPTXIntrinsics.td b/llvm/lib/Target/NVPTX/NVPTXIntrinsics.td
index b2e05a567b4fe..1943e94c3ee7a 100644
--- a/llvm/lib/Target/NVPTX/NVPTXIntrinsics.td
+++ b/llvm/lib/Target/NVPTX/NVPTXIntrinsics.td
@@ -315,7 +315,7 @@ defm MATCH_ALLP_SYNC_64 : MATCH_ALLP_SYNC<Int64Regs, "b64", int_nvvm_match_all_s
 multiclass REDUX_SYNC<string BinOp, string PTXType, Intrinsic Intrin> {
   def : NVPTXInst<(outs Int32Regs:$dst), (ins Int32Regs:$src, Int32Regs:$mask),
           "redux.sync." # BinOp # "." # PTXType # " $dst, $src, $mask;",
-          [(set i32:$dst, (Intrin i32:$src, Int32Regs:$mask))]>,
+          [(set i32:$dst, (Intrin i32:$mask, Int32Regs:$src))]>,
         Requires<[hasPTX<70>, hasSM<80>]>;
 }
 

[durga4github]

The latest changes LGTM

[Artem-B]

@AustinSchuh would you like me to merge the change for you, once the checks are done?

[AustinSchuh]

@AustinSchuh would you like me to merge the change for you, once the checks are done?

That would be wonderful. I don't know how to merge it (happy to learn, but I suspect I won't do it more than a couple of times)

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[Artem-B]

@AustinSchuh would you like me to merge the change for you, once the checks are done?

That would be wonderful. I don't know how to merge it (happy to learn, but I suspect I won't do it more than a couple of times)

https://llvm.org/docs/DeveloperPolicy.html#obtaining-commit-access explains the process. For occasional patches, asking reviewers to land the patches is perfectly fine.