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
46 lines
1.3 KiB
C
46 lines
1.3 KiB
C
// XFAIL:*
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//// We don't yet support DW_OP_implicit_pointer in llvm.
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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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//// Check that 'param' in 'fun' can be read throughout, and that 'pa' and 'pb'
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//// can be dereferenced in the debugger even if we can't provide the pointer
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//// value itself.
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int globa;
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int globb;
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//// A no-inline, read-only function with internal linkage is a good candidate
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//// for arg promotion.
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__attribute__((__noinline__))
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static void use_promote(const int* pa) {
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//// Promoted args would be a good candidate for an DW_OP_implicit_pointer.
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globa = *pa; // DexLabel('s2')
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}
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__attribute__((__always_inline__))
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static void use_inline(const int* pb) {
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//// Inlined pointer to callee local would be a good candidate for an
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//// DW_OP_implicit_pointer.
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globb = *pb; // DexLabel('s3')
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}
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__attribute__((__noinline__))
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int fun(int param) {
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volatile int step = 0; // DexLabel('s1')
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use_promote(¶m);
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use_inline(¶m);
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return step; // DexLabel('s4')
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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='s4')
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// DexExpectWatchValue('*pa', 5, on_line='s2')
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// DexExpectWatchValue('*pb', 5, on_line='s3')
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