https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91869
Bug ID: 91869
Summary: Constant bitfield assignment causes unnecessary use of
memory and instructions
Product: gcc
Version: 9.2.1
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: pdj at knaldgas dot dk
Target Milestone: ---
Host: x86_64
Target: x86_64
Build: x86_64
Created attachment 46914
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=46914&action=edit
MWE in C, compile with gcc -O2
Assigning constant bitfields to variables cause unnecessary use of memory,
instructions and time.
Having a byte consisting of multiple bitfields, setting more than one bitfield
causes gcc to place the constant in .rodata instead of immediate loading.
When all fields are zero, or setting only one of the bitfields, immediate
loading is used as expected.
On microcontrollers with limited memory this is a problem. Bitfields
(representation clauses) in Ada are well-defined, but I've written the MWE in C
as C's bitfields exhibit the same (mis-)behaviour.
The attached code should be generated with:
$ gcc -O2 main.c -o main
$ objdump -h -S main > main.lss
Tried with Targets being x86_64 (9.2.1), arm-linux-gnueabihf (9.2.1) and avr
(9.2.0), same result.
