The attached testcase shows a code generation regression which can be seen on
x86 but I believe it to be target indepedent.
The code is copying data between two structures, both of which contain
bitfields. The source structure uses 1-bit bitfields of type "int", while the
destination structure uses 1-bit bitfields of type "_Bool". At the end of the
tree optimizers, the code looks like this:
params.cliplmt_ch1 = (int) msg.cliplmt_ch1 != 0;
params.cliplmt_ch2 = (int) msg.cliplmt_ch2 != 0;
params.gate_ch1 = (int) msg.gate_ch1 != 0;
params.gate_ch2 = (int) msg.gate_ch2 != 0;
The problem is that the comparison with zero is never removed by the
optimizers. The generated code looks like this:
testb $1, %bl
setne %cl
In 3.4, instead, the generated code looks better:
andl $1, %ebx
Notice this this does not happen in C++ (using a "bool" bitfield instead
of "_Bool"). We get a BITFIELD_REF there:
D.1643 = BIT_FIELD_REF <msg, 8, 40>;
params.cliplmt_ch1 = (D.1643 & 1) != 0;
params.cliplmt_ch2 = (D.1643 & 2) != 0;
params.gate_ch1 = (D.1643 & 4) != 0;
params.gate_ch2 = (D.1643 & 8) != 0;
and the generated code is optimal.
--
Summary: [4.0 Regression] Inefficient code generated for bitfield
assignment
Product: gcc
Version: unknown
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P2
Component: rtl-optimization
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: giovannibajo at libero dot it
CC: aleph at develer dot com,bernie at develer dot com,gcc-
bugs at gcc dot gnu dot org,sayle at gcc dot gnu dot org
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19331
