https://bugs.llvm.org/show_bug.cgi?id=39024
Bug ID: 39024
Summary: [DebugInfo@O2][Dexter] Vectorized code linenos step
through loop even after completion
Product: new-bugs
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Keywords: wrong-debug
Severity: normal
Priority: P
Component: new bugs
Assignee: unassignedb...@nondot.org
Reporter: jeremy.morse.l...@gmail.com
CC: llvm-bugs@lists.llvm.org
Blocks: 38768
In the loop below, when vectorized, compiler generated code gets program line
numbers that have the potential to be misleading. My test setup is using
clang/llvm r342527, compiling "-O2 -g -fno-inline" targeting x86_64, and
running under gdb or lldb.
The code below sums the contents of a buffer (and is completely contrived); it
gets vectorized into a pair of vectorized/scalar loops as illustrated in the
comments here [0]. Awkwardly, instructions in the "middle block", which just
reduce the vector-accumulator registers down into a scalar register, get the
line numbers for a loop iteration. This causes the user to step through part of
the loop again, even if it's finished.
IMHO this is a defect: the middle block is compiler generated code to plumb
around the transformations that have been performed and shouldn't be stepped
through by the developer. It potentially misleads by suggesting that the loop
is still alive, because the developer can still step through part of it.
If one steps through this whole program you get the impression that the body
executes four times and has a different path through line numbers on each
iteration. Obviously vectorization puts a limit on how faithful debug data can
be to the original program, but IHMO this could be better.
[0]
https://github.com/llvm-mirror/llvm/blob/a858f4fe0bec699a58833b6277633b2c1b8f90d4/lib/Transforms/Vectorize/LoopVectorize.cpp#L2567
-------->8--------
#include <stdlib.h>
#include <string.h>
#define BUFSZ 256
int
foo(int argc, int init, int *bar)
{
if (argc + 1 > BUFSZ)
return 0;
int tmp = argc;
int bees;
for (int j = 0; j < argc; j++) {
bees = bar[j];
tmp += bees;
}
return tmp;
}
int
main(int argc, char **argv)
{
int bar[256];
memset(bar, 1, 256);
return foo(16, 1, bar);
}
--------8<--------
Referenced Bugs:
https://bugs.llvm.org/show_bug.cgi?id=38768
[Bug 38768] [meta][DebugInfo] Umbrella bug for poor debug experiences
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