https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105090
Bug ID: 105090
Summary: BFI instructions are not generated on
arm-none-eabi-g++
Product: gcc
Version: 11.2.1
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: andrew.jeddeloh at gmail dot com
Target Milestone: ---
Created attachment 52704
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=52704&action=edit
.ii file for test case
The following code could generate a BFI instruction but does not. Clang will
generate one.
#include <stdint.h>
uint32_t emplace(uint32_t into, uint32_t what) {
constexpr uint32_t shift = 5;
constexpr uint32_t width = 4;
constexpr uint32_t mask = ((1 << width) - 1) << shift;
return (into & ~mask) | ((what << shift) & mask);
}
you can write equivalent C code and get the same problem.
gcc 8.5 and clang generate:
bfi r0, r1, #5, #4
bx lr
Whereas 9.3+ generates:
lsls r1, r1, #5
and r1, r1, #480
bic r0, r0, #480
orrs r0, r0, r1
bx lr
These are compiled with arm-none-eabi-gcc -Wall -Wextra -mcpu=cortex-m4 -O3 -c
Compile output is silent. Attached is .ii
I also tried O2, O1 and Os.
See problem with godbolt: https://godbolt.org/z/57h5Yd9ov
A lot of embedded development involves setting certain chunks of memory mapped
registers like this and would greatly benefit from this being fixed.
GCC version info from my computer:
Using built-in specs.
COLLECT_GCC=arm-none-eabi-gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/arm-none-eabi/11.2.0/lto-wrapper
Target: arm-none-eabi
Configured with: /build/arm-none-eabi-gcc/src/gcc-11.2.0/configure
--target=arm-none-eabi --prefix=/usr --with-sysroot=/usr/arm-none-eabi
--with-native-system-header-dir=/include --libexecdir=/usr/lib
--enable-languages=c,c++ --enable-plugins --disable-decimal-float
--disable-libffi --disable-libgomp --disable-libmudflap --disable-libquadmath
--disable-libssp --disable-libstdcxx-pch --disable-nls --disable-shared
--disable-threads --disable-tls --with-gnu-as --with-gnu-ld --with-system-zlib
--with-newlib --with-headers=/usr/arm-none-eabi/include
--with-python-dir=share/gcc-arm-none-eabi --with-gmp --with-mpfr --with-mpc
--with-isl --with-libelf --enable-gnu-indirect-function
--with-host-libstdcxx='-static-libgcc -Wl,-Bstatic,-lstdc++,-Bdynamic -lm'
--with-pkgversion='Arch Repository' --with-bugurl=https://bugs.archlinux.org/
--with-multilib-list=rmprofile
Thread model: single
Supported LTO compression algorithms: zlib zstd
gcc version 11.2.0 (Arch Repository)
Possibly related but I'm not sure:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85628
