https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106076
Bug ID: 106076
Summary: Sub-optimal code is generated for checking bitfields
via proxy functions
Product: gcc
Version: 13.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: kyrylo.bohdanenko at gmail dot com
Target Milestone: ---
Consider the following struct:
#include <cstdint>
struct SomeClass {
uint16_t dummy1 : 1;
uint16_t cfg2 : 1;
uint16_t cfg3 : 1;
uint16_t dummy2 : 1;
uint16_t dummy3 : 1;
uint16_t dummy4 : 1;
uint16_t cfg1 : 1;
uint16_t dummy5 : 1;
uint16_t cfg4 : 1;
constexpr bool checkA() const { return cfg1 || cfg2 || cfg3; }
constexpr bool checkB() const { return cfg4; }
constexpr bool checkA_B() const { return (cfg1 || cfg2 || cfg3) || cfg4; }
constexpr bool checkA_B_SLOW() const { return checkA() || checkB(); }
};
For the following functions (which do the same thing) GCC generates different
assembly.
bool check(const SomeClass& rt) {
return rt.checkA_B();
}
bool check_SLOW(const SomeClass& rt) {
return rt.checkA_B_SLOW();
}
Compiled as:
g++ -std=c++17 -S
The assembly:
; demangled: check(SomeClass const&)
_Z5checkRK9SomeClass:
endbr64
testw $326, (%rdi)
setne %al
ret
; demangled: check_SLOW(SomeClass const&)
_Z10check_SLOWRK9SomeClass:
endbr64
movzwl (%rdi), %edx
movl $1, %eax
testb $70, %dl
jne .L3
movzbl %dh, %eax
andl $1, %eax
.L3:
ret
As we can see, during check_SLOW GCC decided to check the result on
byte-by-byte basis introducing a conditional jump in between. It looks like GCC
did not fully analyse the code after inlining checkA() and checkB().
FYI, the same code on Clang produces the 1st option of ASM for both functions.
