[LLVMdev] Debugging information problem: code being reordered / debug point jumping around

Matt Pharr matt.pharr at gmail.com
Sun Jul 31 22:49:28 PDT 2011

I'm seeing some undesirable behavior where, when generating debugging information, sometimes the point in the debugger will jump forwards and backwards during single stepping, even though the output from our frontend (this is the ispc compiler), is emitting LLVM instructions with a strictly forward-moving/increasing set of source locations.  I'm wondering if we're doing something wrong here, or if this points to a bug somewhere in LLVM.

I've attached a case that demonstrates this.  Specifically, the instructions in the LLVM assembly language for the foo() function have the following metadata items associated with them, in the following order: 5519, 5520, 5522, 5523, 5524, 5525, 5526, 5527, 5528, 5529, 5530, 5531, 5532, 5533, 5534, 5521.  These correspond to the source file locations: [4,22], [5,5], [5,11], [5,22], [5,23], [6,14], [6,22], [6,24], [6,26], [8,11], [8,15], [9,9], [9,10], [9,11], [10,12], [10,12].  ([line, column]).

However, if I run "llc -O0 debug.ll -o debug.s" with top of tree, the set of source locations indicated in comments jumps from [5,23] to [9,10], and then back to [6,22].  (And if I single-step through this code in gdb, I see the corresponding jump around.)

I'd be happy to hear any comments or suggestions!


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