[cfe-dev] Cortex-M4F Assembly Code Generation Incorrect for float casting, causing HardFaults

[mats petersson via cfe-dev]

Does the C and C++ standards not state that loading a value from an
unaligned addres (e.g. float from an address not aligned to 4) is undefined
behaviour?
I'm not saying SO is the definitive reference for this, but it would seem
like it says so, with a reference to the spec:
https://stackoverflow.com/questions/28893303/is-a-misaligned-load-due-to-a-cast-undefined-behavior

Whilst it may well be that this "works" on some archiectures (particularly
x86), I do believe it's perfectly valid to produce code that assumes
aligned data.

Or am I missing something on the subject?

--
Mats

On 10 August 2017 at 14:50, Dan Walmsley via cfe-dev <cfe-dev at lists.llvm.org
> wrote:

> If have filed a bug here:
>
>
> https://bugs.llvm.org/show_bug.cgi?id=34143
> ------------------------------
> *From:* cfe-dev <cfe-dev-bounces at lists.llvm.org> on behalf of Dan
> Walmsley via cfe-dev <cfe-dev at lists.llvm.org>
> *Sent:* 10 August 2017 13:41:39
> *To:* cfe-dev at lists.llvm.org; Hans Wennborg
> *Subject:* [cfe-dev] Cortex-M4F Assembly Code Generation Incorrect for
> float casting, causing HardFaults
>
>
> Basically the following code can repro the issue:
>
>
> Compile flags are:
>
> Wall -c -fshort-enums -ggdb3 -std=c++14 -fno-rtti -fno-exceptions
> -mfpu=fpv4-sp-d16 -mfloat-abi=hard -mfpu=fpv4-sp-d16 -mfloat-abi=hard
> -ffunction-sections -fdata-sections -O1 -target arm-none-eabi
> -mcpu=cortex-m4 -mthumb
>
>
> code to reproduce is:
>
>
> ```
>
> template <typename T> inline T Read (uint8_t* data)
> {
>     auto valuePtr = reinterpret_cast<T*> (data);
>
>     auto value = *valuePtr;
>
>     return value;
> }
>
> static float test = 180.0f;
>
> int main (void)
> {
>     uint8_t* data = new uint8_t[8];
>
>     memcpy (data, &test, sizeof (float));
>
>     auto value = Read<float> (data);
>
>
>     memcpy (data + 1, &test, sizeof (float));
>
>     value = Read<float> (data + 1); // hard fault here!
>
>     return value;
> }
>
> ```
>
>
> My diagnosis is that when it dereferences the float pointer it uses the
> VLDR instruction, https://developer.arm.com/docs/100069/
> 0607/advanced-simd-instructions-32-bit/vldr
>
>
> This instruction can only be used on word aligned boundaries.
>
>
> So It works for first call to my Read method, but the second call which is
> not aligned, it crashes.
>
>
> Not sure who is the right person to fix this.
>
>
> I will reply with  a bug, once I manage to login to the bug database.
> Thanks
>
> Dan
>
>
>
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>
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