https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94952
Bug ID: 94952
Summary: Possible false positive of uninitialized variable
usage during release build in
gimple-ssa-store-merging.c
Product: gcc
Version: 10.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: stefansf at linux dot ibm.com
Target Milestone: ---
Created attachment 48450
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=48450&action=edit
initialize variables `bitpos`, `bitregion_start`, and `bitregion_end`
While bootstrapping GCC on S/390 with --enable-checking=release the following
warnings are raised:
gcc/gimple-ssa-store-merging.c: In member function 'virtual unsigned int
{anonymous}::pass_store_merging::execute(function*)':
gcc/gimple-ssa-store-merging.c:1412:24: error:
'bitregion_end.poly_int_pod<1, long unsigned int>::coeffs[0]' may be used
uninitialized in this fu
nction [-Werror=maybe-uninitialized]
1412 | , ops { op0r, op1r }
| ^
gcc/gimple-ssa-store-merging.c:4672:32: note: 'bitregion_end.poly_int_pod<1,
long unsigned int>::coeffs[0]' was declared here
4672 | poly_uint64 bitregion_start, bitregion_end;
| ^~~~~~~~~~~~~
gcc/gimple-ssa-store-merging.c:1412:24: error: 'bitregion_start.poly_int_pod<1,
long unsigned int>::coeffs[0]' may be used uninitialized in this
function [-Werror=maybe-uninitialized]
1412 | , ops { op0r, op1r }
| ^
gcc/gimple-ssa-store-merging.c:4672:15: note: 'bitregion_start.poly_int_pod<1,
long unsigned int>::coeffs[0]' was declared here
4672 | poly_uint64 bitregion_start, bitregion_end;
| ^~~~~~~~~~~~~~~
In file included from gcc/coretypes.h:449,
from gcc/gimple-ssa-store-merging.c:143:
gcc/poly-int.h:2063:21: error: 'bitpos.poly_int<1, long unsigned
int>::<anonymous>.poly_int_pod<1, long unsigned int>::coeffs[0]' may be used
uni
nitialized in this function [-Werror=maybe-uninitialized]
2063 | if (a.coeffs[i] % b != 0)
| ~~~~~~~~~~~~^~~
gcc/gimple-ssa-store-merging.c:4671:24: note: 'bitpos.poly_int<1, long unsigned
int>::<anonymous>.poly_int_pod<1, long unsigned int>::coeffs[0]'
was declared here
4671 | poly_uint64 bitsize, bitpos;
| ^~~~~~
Function `mem_valid_for_store_merging` initializes the aforementioned variables
`bitpos`, `bitregion_start`, and `bitregion_end` if it returns a value
different than `NULL_TREE`.
tree base_addr
= mem_valid_for_store_merging (lhs, &bitsize, &bitpos,
&bitregion_start, &bitregion_end);
Thus the local variable `invalid` equals `true` in case the aforementioned
variables are uninitialized.
bool invalid = (base_addr == NULL_TREE
|| /* ... */);
What follows is a check on `invalid`:
if (invalid)
;
else if (rhs_valid_for_store_merging_p (rhs))
{
// ...
}
else if (TREE_CODE (rhs) != SSA_NAME)
invalid = true;
else
{
// ... (*)
}
// ...
if (invalid
|| /* ... */)
return terminate_all_aliasing_chains (NULL, stmt);
In total we have that in case `bitpos`, `bitregion_start`, or `bitregion_end`
is uninitialized, then `invalid` equals true and the function returns prior an
access which renders the warnings as false positives. Interestingly, if (*)
gets removed which cannot be taken in case the variables are uninitialized,
then the warnings disappear which endorses the suspicion of a false positive.
The attached patch fixes the warnings by initializing `bitpos`,
`bitregion_start`, or `bitregion_end` to zero.
