[clang] [libcxx] [clang] Add builtin to clear padding bytes (prework for P0528R3) (PR #75371)

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huixie90 wrote:

> Bitfield load and store operations should be done using the same offset/size we normally use to access the bitfield; unconditionally using byte load/store operations will impair optimizations/performance. I guess this might not be possible when unions are involved, but it shouldn't be that hard for the non-union cases.
> 
> The format of builtin-clear-padding-codegen.cpp seems mostly fine, but consider using update_cc_test_checks.py to automate writing the CHECK lines. Please add a couple tests for empty classes and unions.
> 
> A few comments in the code outlining how the recursion and the interval representation work would be helpful.

Thanks very much for your review. and really sorry it took me more than a year to come back to this.

> unconditionally using byte load/store operations will impair optimizations/performance.

If you still remember this comment, is it referring to the final "clearing padding step", where I zeroing bytes-by-bytes? If so, apologies for not being familiar with this, what would be the best way of achieving it? So my current approach is 
- Visit recursively to figure out all the bits ranges that data occupied
- figure out all the holes (padding)
- generate storing zero bytes-by-bytes for the wholes bytes and bits

on the last step, for non-bitfield, i was basically doing

```cpp
              Address ElementAddr(Element, CGF.Int8Ty, CharUnits::One());
              CGF.Builder.CreateStore(Zero, ElementAddr);
```

for bitfield, i was basically doing

```cpp
          uint8_t mask = ((1 << EndBit) - 1) & ~((1 << StartBit) - 1);
          auto *MaskValue = ConstantInt::get(CGF.Int8Ty, mask);
          auto *NewValue = CGF.Builder.CreateAnd(Value, MaskValue);
```

This might not be the most optimised way of doing this. however, I am not familiar with this part of the code what would be the alternative.

> Bitfield load and store operations should be done using the same offset/size we normally use to access the bitfield;

Hmm, the puzzle I have is that I am not loading/storing the BitField themselves, but the paddings around them, which may or may not be occupied by other stuff.


> The format of builtin-clear-padding-codegen.cpp seems mostly fine, but consider using update_cc_test_checks.py to automate writing the CHECK lines. Please add a couple tests for empty classes and unions.

Absolutely, thanks for pointing out to update_cc_test_checks.py .  I was mainly testing using our libc++ test suites and was not sure how to automatically generate these IR codegen tests. will update the test to cover all the cases.


https://github.com/llvm/llvm-project/pull/75371