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Fix compilation error on gcc 9.1 #441

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Apr 20, 2020
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Fix compilation error on gcc 9.1 #441

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6 changes: 4 additions & 2 deletions lib/mayaUsd/render/vp2RenderDelegate/mesh.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -164,7 +164,8 @@ namespace {
}

if (channelOffset == 0 && sizeof(DEST_TYPE) == sizeof(SRC_TYPE)) {
memcpy(vertexBuffer, primvarData.cdata(), sizeof(DEST_TYPE) * numVertices);
const void* source = static_cast<const void*>(primvarData.cdata());
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I'm not sure I understand this fix. Isn't the compiler error telling us that sizeof test somehow failed and we are using memcpy for not matching types?

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No. gcc 9.1 is only complaining about the type mismatch between source and destination of memcpy op. The memcpy function won't called if their types don't match, but this is run-time behavior that won't be taken into account at compile time.

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I may be missing something here and we would need to have a bit more information about which actual invocation of _FillPrimvarData is generating this error.

FWIW. There is one possibility where we will have a destination as Vec4f and source as Vec3f when filling color with alpha buffer. In this case, we should never enter the memcpy section...and the error should never be generated.

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Indeed the invocation is from where the source is Vec3f and the destination is Vec4f. And we indeed don't enter the memcpy section at run time. But the compiler won't examine run-time code path at compile time, that is why it still generate the error.

The fix has been validated on gcc 9.1.

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One detail, sizeof is a constant expression, the compiler should know that it doesn't enter this condition. Maybe it's a bug in the compiler and this is why we never saw it before.

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I guess even if we had this memcpy call inside a if(false) clause the compiler would still generate this error. More because of compiler strictness, I am not sure if it is a bug.

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Looking at C++17 and what if const expression can do...you may be right that compiler won't help us here because of value dependency.

OK, you convinced me. Thx.

memcpy(vertexBuffer, source, sizeof(DEST_TYPE) * numVertices);
}
else {
for (size_t v = 0; v < numVertices; v++) {
Expand Down Expand Up @@ -244,7 +245,8 @@ namespace {
}

if (channelOffset == 0 && sizeof(DEST_TYPE) == sizeof(SRC_TYPE)) {
memcpy(vertexBuffer, primvarData.cdata(), sizeof(DEST_TYPE) * numVertices);
const void* source = static_cast<const void*>(primvarData.cdata());
memcpy(vertexBuffer, source, sizeof(DEST_TYPE) * numVertices);
}
else {
for (size_t v = 0; v < numVertices; v++) {
Expand Down