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Fix UB in allgather with zero-size output #495

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Fix UB in allgather with zero-size output #495

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9 changes: 7 additions & 2 deletions gloo/allgather.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -44,6 +44,11 @@ void allgather(AllgatherOptions& opts) {
const size_t inBytes = out->size / context->size;
const size_t outBytes = out->size;

// Short circuit if the output is empty.
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@niyazmurshed niyazmurshed Mar 12, 2026

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will this be better before calculating sendRank & recvRank i.e under sanity checks?

if (outBytes == 0) {
return;
}

// If the input buffer is specified, this is NOT an in place operation,
// and the output buffer needs to be primed with the input.
if (in != nullptr) {
Expand All @@ -53,8 +58,8 @@ void allgather(AllgatherOptions& opts) {
in->size);
}

// Short circuit if there is only a single process or the output is empty.
if (context->size == 1 || outBytes == 0) {
// Short circuit if there is only a single process.
if (context->size == 1) {
return;
}

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