[clang] [Clang][Windows] Preserve x86_fp80 and vector alignment under #pragma pack. (PR #208256)

[Zahira Ammarguellat via cfe-commits]

zahiraam wrote:

To verify the behavior with `SSE` disabled, I compiled a test case forcing pure `x87` code generation:

```
typedef long double x87_type;

#pragma pack(push, 8)
struct PackedX87 {
    x87_type val;
};
#pragma pack(pop)

void test(PackedX87 *ptr) {
    ptr->val = 3.14L;
}
```

Even though the generated assembly uses pure `x87` instructions:

```
fldt    .LCPI0_0(%rip)    # x87 load
fstpt   (%rax)            # x87 store
```
The target datalayout specifies what the ABI requires (even without this patch):
`target datalayout = "e-m:w-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128"`

The compiler now enforces `16-byte` alignment in the IR:
`store x86_fp80 3.140000e+00, ptr %val, align 16`

and pads constant pools to `16 bytes` (even without this patch):
```
.LCPI0_0:
    .quad   0xc8f5c28f5c28f5c3
    .short  0x4000
    .zero   6              # Explicit padding to 16 bytes
```


This demonstrates that the `16-byte` alignment is an `ABI` requirement, not an instruction requirement.  The target datalayout `f80:128` has always specified that `x86_fp80` requires 16-byte alignment as part of the ABI.
 
So, yes, we're enforcing `16-byte` alignment even though `x87` instructions don't strictly need it, because:
- The ABI requires it for interoperability.
- It prevents crashes with SSE-optimized code paths.
- It ensures consistent struct layouts across compilation units.
- Conservative alignment (16 bytes) works correctly with both x87 AND SSE.

https://github.com/llvm/llvm-project/pull/208256