[Bug middle-end/56362] New: bitfield refs over-optimized?
jay.krell at cornell dot edu
gcc-bugzilla@gcc.gnu.org
Sun Feb 17 10:28:00 GMT 2013
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56362
Bug #: 56362
Summary: bitfield refs over-optimized?
Classification: Unclassified
Product: gcc
Version: 4.7.2
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
AssignedTo: unassigned@gcc.gnu.org
ReportedBy: jay.krell@cornell.edu
Our front end is a bit wierd.
We don't declare the fields of our structs.
We use bitfield refs to pick out the fields we know are there.
Something like this:
If in C you had the reasonable:
struct { int a,b,c,d } e;
e.b => component_ref
We have:
struct /* size 16 bytes */ e;
bitfield_ref(e, offset 4 bytes, size 4 bytes, type int)
I have changed it so sometimes, instead:
*(int*)(&e + 4)
e.g. when reading floating point fields,
but that defeats optimizations more.
so, then, while this mostly works, and generates better
code than the pointer offset + deref form, it does seem to
very occasionally not work.
I have not fully debugged this, at least not in years.
4.7.2/gcc/fold-const.c has this code:
/* A bit-field-ref that referenced the full argument can be stripped. */
if (INTEGRAL_TYPE_P (TREE_TYPE (arg0))
&& TYPE_PRECISION (TREE_TYPE (arg0)) == tree_low_cst (arg1, 1)
&& integer_zerop (op2))
return fold_convert_loc (loc, type, arg0);
I believe this is a bit too aggressive.
Such as when there is a sign extension implied.
I added these two conditions to make to make it less aggressive:
&& INTEGRAL_TYPE_P (type)
&& TYPE_UNSIGNED (type) == TYPE_UNSIGNED (TREE_TYPE (arg0))
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