Tweak dexter-tests/memvars/inline-escaping-function.c added in D94761
(b7e516202eb6) by adding a 'param' use after the merge point. The test XFAILS
with and without this change, but without it the test looks very similar to
memvars/unused-merged-value.c. The test now demonstrates the problem more
clearly.
These dexter tests illustrate PR48719, the summary of which is:
Sometimes we insert dbg.values for merged values (PHIs) when promoting
variables, sometimes we don't. Sometimes there is no PHI because the merged
value is never used. It doesn't matter because LiveDebugValues understands these
merged values (implicit or otherwise) and correctly updates the debug
info. Importantly, these merged variable values (which may or may not exist as
PHIs, and may or not be represented with dbg.values) are //always// implicitly
defined by the combination of incoming edges and the incoming variable locations
along those edges by virtue of LiveDebugValues existing. Unfortunately, it is
possible to mess with the CFG and remove / move these edges before
LiveDebugValues runs. In this case our debug info model only works when the
merged value is tracked by a dbg.value. Currently, this is only done rigorously
for variables which are A) promoted in the first round of mem2reg and B) are
used after the merge point.
As an example, compile the following source with -O3 -g and step through with a
debugger. You will see parama=5 throughout the function fun which is incorrect -
we expect to see param=20 after the conditional assignment.
__attribute__((optnone))
void esc(int* p) {}
__attribute__((optnone))
void fluff() {}
__attribute__((noinline))
int fun(int parama, int paramb) {
if (parama)
parama = paramb;
fluff(); // DexLabel('s0')
esc(¶ma);
return 0;
}
int main() {
return fun(5, 20);
}
1. parama is escaped by esc(¶ma) so it is not promoted by
SROA/mem2reg (failing condition "A" above).
2. InstCombine's LowerDbgDeclare converts the dbg.declare to a set of
dbg.values (tracking the stored SSA values).
3. InstCombine replaces the two stores to parama's alloca (the initial
parameter register store in entry and the assignment in if.then) with a
PHI+store in the common sucessor.
4. SimplifyCFG folds the blocks together and converts the PHI to a
select.
The debug info is not updated to account for the merged value in the successor
prior to SimplifyCFG when it exists as a PHI, or during when it becomes a
select.
As with D89543, which added some dexter tests for escaped locals, the idea is
to build a set of source-level tests which highlights existing issues and
might be useful in evaluating a new debug info model.
Reviewed By: rnk
Differential Revision: https://reviews.llvm.org/D94761
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
