https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105973
Bug ID: 105973
Summary: Wrong branch prediction for if (COND) { if(x)
noreturn1(); else noreturn2(); }
Product: gcc
Version: 13.0
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: redi at gcc dot gnu.org
Target Milestone: ---
Given this code:
__attribute__((noreturn)) void throw1();
__attribute__((noreturn)) void throw2();
typedef decltype(sizeof(0)) size_t;
#if defined LIKELY
# define PREDICT(C) __builtin_expect(C,1)
#elif defined UNLIKELY
# define PREDICT(C) __builtin_expect(C,0)
#else
# define PREDICT(C) (C)
#endif
template<typename T>
T* allocate(size_t n)
{
if (PREDICT(n > (__PTRDIFF_MAX__ / sizeof(T))))
{
if (n > (__SIZE_MAX__ / sizeof(T)))
throw1();
throw2();
}
return (T*) ::operator new(n * sizeof(T));
}
int* alloc_int(size_t n)
{
return allocate<int>(n);
}
The condition decorated with PREDICT is compiled to different code with
-DLIKELY and -DUNLIKELY, as expected.
However with neither macro defined, the result is the same as -DLIKELY (for any
optimization level > -O0). i.e. the calls to throw1 and throw1 come first and
the return statement requires a branch:
_Z9alloc_intm:
.LFB1:
.cfi_startproc
movq %rdi, %rax
shrq $61, %rax
je .L2
subq $8, %rsp
.cfi_def_cfa_offset 16
shrq $62, %rdi
je .L3
call _Z6throw1v
.p2align 4,,10
.p2align 3
.L3:
call _Z6throw2v
.p2align 4,,10
.p2align 3
.L2:
.cfi_def_cfa_offset 8
salq $2, %rdi
jmp _Znwm
.cfi_endproc
Surely this is wrong?
If calling a noreturn function is considered unlikely, then surely entering a
block that always calls a noreturn function should also be unlikely?
Clang gets this right, generating the same code as UNLIKELY by default, and
only requiring a branch for the return value when LIKELY is defined.
This code is reduced from std::allocator in libstdc++ and I thought I should be
able to remove a redundant __builtin_expect, but it's needed due to this.
