66d03af88c
Recently there has been renewed interest in improving debug-info for variables that (partially or otherwise) live on the stack in optimised code. At the moment instcombine speculates that stack slots are probably going to be promoted to registers, and prepares the debug-info accordingly. It runs a function called LowerDbgDeclare which converts dbg.declares to a set of dbg.values after loads, and before stores and calls. Sometimes the stack location remains (e.g. for escaped locals). If any dbg.values become undef where the stack location is still valid we end up unnecessarily reducing variable location coverage due to our inability to track multiple locations simultaneously. There is a flag to disable this feature (-instcombine-lower-dbg-declare=0), which prevents this conversion at the cost of sometimes providing incorrect location info in the face of DSE, DCE, GVN, CSE etc. This has been discussed fairly extensively on PR34136. The idea of these tests is to provide examples of situations that we should consider when designing a new system, to aid discussions and eventually help evaluate the implementation. Dexter isn't ideal for observing specific optimisation behaviour. Writing an exaustive test suite would be difficult, and the resultant suite would be fragile. However, I think having some concrete executable examples is useful at least as a reference. Differential Revision: https://reviews.llvm.org/D89543
53 lines
1.6 KiB
C
53 lines
1.6 KiB
C
// XFAIL:*
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//// Suboptimal coverage, see inlined comments.
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// REQUIRES: lldb
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// UNSUPPORTED: system-windows
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// RUN: %dexter --fail-lt 1.0 -w --debugger lldb \
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// RUN: --builder 'clang-c' --cflags "-O3 -glldb" -- %s
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//// Adapted from https://bugs.llvm.org/show_bug.cgi?id=34136#c4
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int g;
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__attribute__((__noinline__))
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void esc(int* p) {
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g = *p;
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*p = 5;
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}
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__attribute__((__noinline__))
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void thing(int x) {
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g = x;
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}
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__attribute__((__noinline__))
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int fun(int param) {
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esc(¶m); //// alloca is live until here DexLabel('s1')
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if (param == 0) { //// end of alloca live range
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//// param is now a constant, but without lowering to dbg.value we can't
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//// capture that and would still point to the stack slot that may even have
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//// been reused by now.
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////
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//// Right now we get suboptimal coverage for x86: the param load below is
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//// CSE'd with the if condition.
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//// Instcombine runs LowerDbgDeclare and inserts a dbg.value after the load.
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//// SelectionDAG combines the load and cmp. We go from this IR:
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//// %0 = load i32, i32* %param.addr, align 4, !dbg !42, !tbaa !20
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//// call void @llvm.dbg.value(metadata i32 %0, ...
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//// %cmp = icmp eq i32 %0, 0, !dbg !44
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//// to this MIR:
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//// DBG_VALUE $noreg, $noreg, !"param"...
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//// CMP32mi8 %param.addr, 1, $noreg, 0, $noreg, 0, implicit-def $eflags, debug-location !44
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thing(param);
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}
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return 0; // DexLabel('s2')
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}
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int main() {
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return fun(5);
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}
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// DexExpectWatchValue('param', '5', from_line='s1', to_line='s2')
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