https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83555
Bug ID: 83555
Summary: Unnecessary null check when static_cast is used with
references.
Product: gcc
Version: 7.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: maxim.yegorushkin at gmail dot com
Target Milestone: ---
When the following code is compiled with `g++ --std=c++14 -O3`:
struct A { int a; };
struct B { int b; };
struct C : A, B { int c; };
C* f(B* b) { return static_cast<C*>(b); }
C* g(B* b) { return &static_cast<C&>(*b); }
It generates the following x86-64 code:
f(B*):
leaq -4(%rdi), %rax
testq %rdi, %rdi
movl $0, %edx
cmove %rdx, %rax
ret
g(B*):
leaq -4(%rdi), %rax
testq %rdi, %rdi
movl $0, %edx
cmove %rdx, %rax
ret
Function f contains the check for null pointer as expected because static_cast
is required to preserve it. Whereas function g contains the unnecessary check
for null because the reference cannot be null.
I also tried compiling the code with clang-5.0.0 and it correctly omits the
null check in function g generating the expected assembly:
g(B*):
leaq -4(%rdi), %rax
retq
Why does g++ generate that unnecessary null check in function g please, even
when compiled with -fno-delete-null-pointer-checks?
