[Bug tree-optimization/105420] Bogus -Warray-bounds with non-compile time-constant variable

[byteslice at airmail dot cc via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105420

--- Comment #2 from Liam White  ---
Created attachment 52906
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=52906=edit
Preprocessed source

The first attachment is automatically and manually reduced from the following
source:

ResultCode KThread::Initialize(KThreadFunction func, uintptr_t arg, VAddr
user_stack_top, s32 prio,
   s32 virt_core, KProcess* owner, ThreadType type)
{
// Assert parameters are valid.
ASSERT((type == ThreadType::Main) || (type == ThreadType::Dummy) ||
   (Svc::HighestThreadPriority <= prio && prio <=
Svc::LowestThreadPriority));
ASSERT((owner != nullptr) || (type != ThreadType::User));
ASSERT(0 <= virt_core && virt_core <
static_cast(Common::BitSize()));

// Convert the virtual core to a physical core.
const s32 phys_core = Core::Hardware::VirtualToPhysicalCoreMap[virt_core];
ASSERT(0 <= phys_core && phys_core <
static_cast(Core::Hardware::NUM_CPU_CORES));

// ...
}

And it produces this compiler error (compile this preprocessed source with -O2
-Werror=array-bounds -std=gnu++20):

In static member function ‘static constexpr _Tp& std::__array_traits<_Tp,
_Nm>::_S_ref(const _Tp (&)[_Nm], std::size_t) [with _Tp = int; long unsigned
int _Nm = 64]’,
inlined from ‘constexpr const std::array<_Tp, _Nm>::value_type&
std::array<_Tp, _Nm>::operator[](size_type) const [with _Tp = int; long
unsigned int _Nm = 64]’ at k_thread.cpp:59748:25,
inlined from ‘ResultCode
Kernel::KThread::Initialize(Kernel::KThreadFunction, uintptr_t, VAddr, s32,
s32, Kernel::KProcess*, Kernel::ThreadType)’ at k_thread.cpp:102280:77:
k_thread.cpp:59638:36: error: array subscript 64 is above array bounds of
‘std::__array_traits::_Type’ {aka ‘const int [64]’}
[-Werror=array-bounds]
59638 |   { return const_cast<_Tp&>(__t[__n]); }
  | ~~~^
k_thread.cpp: In member function ‘ResultCode
Kernel::KThread::Initialize(Kernel::KThreadFunction, uintptr_t, VAddr, s32,
s32, Kernel::KProcess*, Kernel::ThreadType)’:
k_thread.cpp:77557:51: note: while referencing
‘Core::Hardware::VirtualToPhysicalCoreMap’
77557 | constexpr std::array()>
VirtualToPhysicalCoreMap{
  |  
^~~~
cc1plus: some warnings being treated as errors

I generally believe that this is a bug in the compiler as the array is
similarly never accessed with the constant value 64.

[Bug tree-optimization/105420] Bogus -Warray-bounds with non-compile time-constant variable

[rguenth at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105420

Richard Biener  changed:

   What|Removed |Added

 Ever confirmed|0   |1
   Keywords||diagnostic
 Blocks||56456
   Last reconfirmed||2022-04-29
 Status|UNCONFIRMED |NEW

--- Comment #1 from Richard Biener  ---
GCC threads the Initialize_type check and effectively sees

  if (Initialize_type)
{
  if (virt_core >= 0)
return;
  int phys_core = VirtualToPhysicalCoreMap[virt_core]; // virt_core is
negative
  if (phys_core)
external();
}
  else
{
   int phys_core = VirtualToPhysicalCoreMap[virt_core];
   if (phys_core && Initialize_type == Kernel)
 external();
}

it should probably warn that virt_core is negative, not that it is literally -1
here.

I agree the diagnostic can be improved, but telling the user the execution
path considered is probably difficult (at best we might somehow be able to
emit a maybe warning).

Still there's an obvious defect in your program that's worth fixing, possibly
by refactoring of this code.


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56456
[Bug 56456] [meta-bug] bogus/missing -Warray-bounds