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* [Bug tree-optimization/105420] New: Bogus -Warray-bounds with non-compile time-constant variable
@ 2022-04-28 13:21 byteslice at airmail dot cc
2022-04-29 6:26 ` [Bug tree-optimization/105420] " rguenth at gcc dot gnu.org
2022-04-29 11:42 ` byteslice at airmail dot cc
0 siblings, 2 replies; 3+ messages in thread
From: byteslice at airmail dot cc @ 2022-04-28 13:21 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105420
Bug ID: 105420
Summary: Bogus -Warray-bounds with non-compile time-constant
variable
Product: gcc
Version: 12.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: byteslice at airmail dot cc
Target Milestone: ---
Created attachment 52894
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=52894&action=edit
Reduced example
On gcc 12.0.1 20220413 (Fedora 36 Beta), with c++ -O1 -fexpensive-optimizations
-ftree-vrp -Werror=array-bounds, the attachment fails to compile, with the
following message:
<source>: In function 'void Initialize(int)':
<source>:9:53: error: array subscript -1 is below array bounds of 'int [8]'
[-Werror=array-bounds]
9 | int phys_core = VirtualToPhysicalCoreMap[virt_core];
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^
<source>:2:5: note: while referencing 'VirtualToPhysicalCoreMap'
2 | int VirtualToPhysicalCoreMap[8];
| ^~~~~~~~~~~~~~~~~~~~~~~~
cc1plus: some warnings being treated as errors
Compiler returned: 1
virt_core does not have a compile time-constant evaluation, so this warning is
bogus. The bogus warning does not appear in older versions of GCC.
Removing either -fexpensive-optimizations or -ftree-vrp allows compilation to
succeed.
^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug tree-optimization/105420] Bogus -Warray-bounds with non-compile time-constant variable
2022-04-28 13:21 [Bug tree-optimization/105420] New: Bogus -Warray-bounds with non-compile time-constant variable byteslice at airmail dot cc
@ 2022-04-29 6:26 ` rguenth at gcc dot gnu.org
2022-04-29 11:42 ` byteslice at airmail dot cc
1 sibling, 0 replies; 3+ messages in thread
From: rguenth at gcc dot gnu.org @ 2022-04-29 6:26 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105420
Richard Biener <rguenth at gcc dot gnu.org> 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 <rguenth at gcc dot gnu.org> ---
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
^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug tree-optimization/105420] Bogus -Warray-bounds with non-compile time-constant variable
2022-04-28 13:21 [Bug tree-optimization/105420] New: Bogus -Warray-bounds with non-compile time-constant variable byteslice at airmail dot cc
2022-04-29 6:26 ` [Bug tree-optimization/105420] " rguenth at gcc dot gnu.org
@ 2022-04-29 11:42 ` byteslice at airmail dot cc
1 sibling, 0 replies; 3+ messages in thread
From: byteslice at airmail dot cc @ 2022-04-29 11:42 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105420
--- Comment #2 from Liam White <byteslice at airmail dot cc> ---
Created attachment 52906
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=52906&action=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<s32>(Common::BitSize<u64>()));
// 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<s32>(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<int, 64>::_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<s32, Common::BitSize<u64>()>
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.
^ permalink raw reply [flat|nested] 3+ messages in thread
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2022-04-28 13:21 [Bug tree-optimization/105420] New: Bogus -Warray-bounds with non-compile time-constant variable byteslice at airmail dot cc
2022-04-29 6:26 ` [Bug tree-optimization/105420] " rguenth at gcc dot gnu.org
2022-04-29 11:42 ` byteslice at airmail dot cc
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