Move shadow space reservation to x86_64 makecontext

[ysbaddaden]

This has been puzzling me for a while, and working on #15409 I finally took the time to investigate.

The shadow space (or home space) is a requirement of the x64 call convention: we must reserve 32 bytes before the return address on the stack. It only applies to x64, not to arm64 for example.

We don't have to deal with this when creating the stack. This is a consideration for each individual makecontext to handle.

Explanation:

The old version for win32:

1. aligned the ptr to 16 bytes
2. decremented the ptr by 6 pointer-size (8 bytes): 1 to move it into the stack area (8 bytes), 4 for the shadow space (32 bytes), and 1 last to keep the 16-bytes alignment (another 8 bytes);
3. aligned the ptr to 16 bytes (not needed).

The new version (actually the old non-win32 version):

1. decrements the ptr by 1 pointer-size to move it back into the stack area;
2. aligns the ptr to 16 bytes.

In both versions stack_ptr point to the last first (the stack grows down) addressable Void* in the stack and is aligned to 16 bytes.

Then the new x86_64-microsoft variant of makecontext reserves 32 bytes of shadow space (still aligned to 16 bytes) and the rest of makecontext is still aligned to 16 bytes.

References:

• Microsoft: x64 call convention
• Shadow Space and Stack Alignment (x64)
• Microsoft: arm64 call convention