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Suppress debug info on the AppendTypesSlow trampoline variable #218

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aaronj0 merged 1 commit into from
Jul 23, 2026
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Suppress debug info on the AppendTypesSlow trampoline variable #218

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5 changes: 4 additions & 1 deletion clingwrapper/src/clingwrapper.cxx
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Original file line number Diff line number Diff line change
Expand Up @@ -541,7 +541,10 @@ bool Cppyy::AppendTypesSlow(const std::string& name,

for (const std::string& candidate : candidates) {
std::string var = "__Cppyy_s" + std::to_string(struct_count++);
if (!Cpp::Declare(("__Cppyy_AppendTypesSlow<" + candidate + "> " + var + ";\n").c_str(), /*silent=*/true)) {
// nodebug: with -g the variable's debug info would carry the full DIE
// tree of every template argument (all member declarations included) --
// a large, uncacheable per-lookup cost on heavyweight types.
if (!Cpp::Declare(("__Cppyy_AppendTypesSlow<" + candidate + "> __attribute__((nodebug)) " + var + ";\n").c_str(), /*silent=*/true)) {

@vgvassilev vgvassilev Jul 21, 2026

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That causes the wrappers to be bigger and thus memory hoarding of already a heavy operation. Is that a huge problem for the debug setup?

@conrade-ctc conrade-ctc Jul 23, 2026

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It's a large, measured problem: with -g, each probe variable's debug info pulls the complete DIE tree of its template argument (all member declarations, transitively) into the PTU — on our production replay (large-header trading workload, heavy AppendTypesSlow traffic) that was a 10x wall-clock regression, 1378s → 137s after suppressing, plus the DWARF itself hoarded per lookup. Against that, the attribute adds 26 bytes to a per-probe source buffer that already exists — paid on non-debug setups too, but it's noise next to the buffer itself. If even that bothers you I can gate the attribute text on whether the interpreter session has debug info enabled, at the cost of a slightly less uniform probe string.

@vgvassilev vgvassilev Jul 23, 2026

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In that case, we can cheat on the JitCall side where just after we parse the prototype, we can check if we passed -g to the interpreter, and then call setAttr(createAttr(nodebug)) on wrapper declaration. That would save bytes I believe.

@vgvassilev vgvassilev Jul 23, 2026

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No wait, on a second thought that might be suboptimal. Let's move forward with the current approach -- it would give us one more incentive to get rid of this string manipulations..

TCppType_t varN =
Cpp::GetVariableType(Cpp::GetNamed(var.c_str(), /*parent=*/nullptr));
TCppScope_t instance_class = Cpp::GetScopeFromType(varN);
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