On 09/14/2018 03:35 PM, Jeff Law wrote:
On 9/12/18 11:46 AM, Martin Sebor wrote:
On 08/31/2018 04:07 AM, Richard Biener wrote:
On Thu, Aug 30, 2018 at 7:39 PM Martin Sebor <mse...@gmail.com> wrote:
On 08/30/2018 11:22 AM, Richard Biener wrote:
On August 30, 2018 6:54:21 PM GMT+02:00, Martin Sebor
<mse...@gmail.com> wrote:
On 08/30/2018 02:35 AM, Richard Biener wrote:
On Thu, Aug 30, 2018 at 2:12 AM Martin Sebor <mse...@gmail.com>
wrote:
The attached patch adds code to work harder to determine whether
the destination of an assignment involving MEM_REF is the same
as the destination of a prior strncpy call. The included test
case demonstrates when this situation comes up. During ccp,
dstbase and lhsbase returned by get_addr_base_and_unit_offset()
end up looking like this:
"During CCP" means exactly when? The CCP lattice tracks copies
so CCP should already know that _1 == _8. I suppose during
substitute_and_fold then? But that replaces uses before folding
the stmt.
Yes, when ccp_finalize() performs the final substitution during
substitute_and_fold().
But then you shouldn't need the loop but at most look at the pointer
SSA Def to get at the non-invariant ADDR_EXPR.
I don't follow. Are you suggesting to compare
SSA_NAME_DEF_STMT (dstbase) to SSA_NAME_DEF_STMT (lhsbase) for
equality? They're not equal.
No.
The first loop iterates once and retrieves
1. _8 = &pb_3(D)->a;
The second loop iterates three times and retrieves:
1. _1 = _9
2. _9 = _8
3. _8 = &pb_3(D)->a;
How do I get from _1 to &pb_3(D)->a without iterating? Or are
you saying to still iterate but compare the SSA_NAME_DEF_STMT?
I say you should retrieve _8 = &pb_3(D)->a immediately since the
copies should be
propagated out at this stage.
The warning is issued as the strncpy call is being folded (during
the dom walk in substitute_and_fold_engine::substitute_and_fold)
but before the subsequent statements have been folded (during
the subsequent loop to eliminate statements). So at the point
of the strncpy folding the three assignments above are still
there.
I can't think of a good way to solve this problem that's not
overly intrusive. Unless you have some suggestions for how
to deal with it, is the patch okay as is?
In what pass do you see the the naked copies? In general those should
have been propagated away.
As I said above, this happens during the dom walk in the ccp
pass:
substitute_and_fold_dom_walker walker (CDI_DOMINATORS, this);
walker.walk (ENTRY_BLOCK_PTR_FOR_FN (cfun));
The warning is issued during the walker.walk() call as
strncpy is being folded into memcpy. The prior assignments are
only propagated later, when the next statement after the strncpy
call is reached. It happens in
substitute_and_fold_dom_walker::before_dom_children(). So during
the strncpy folding we see the next statement as:
MEM[(struct S *)_1].a[n_7] = 0;
After the strncpy call is transformed to memcpy, the assignment
above is transformed to
MEM[(struct S *)_8].a[3] = 0;
If they're only discovered as copies within the pass where you're trying
to issue the diagnostic, then you'd want to see if the pass has any
internal structures that tell you about equivalences.
I don't know if this is possible. I don't see any APIs in
tree-ssa-propagate.h that would let me query the internal data
somehow to find out during folding (when the warning is issued).
Martin
