[PATCH] D32205: X86RegisterInfo: eliminateFrameIndex: Force SP in AfterFPPop case; NFC

[Reid Kleckner via Phabricator via llvm-commits]

rnk added inline comments.


================
Comment at: lib/Target/X86/X86RegisterInfo.cpp:679
+  if (AfterFPPop) {
+    assert(!hasBasePointer(MF));
+    assert(!needsStackRealignment(MF));
----------------
MatzeB wrote:
> rnk wrote:
> > Why does this assert hold true? Do we not support tail calls when doing stack realignment?
> I hope we do not do create tail calls that load their target address from the stack in the realignment cases. If we do, then there is a high chance the existing code would miscompile as the frame/base pointer are already reset at the pointer of the tailcall I think.
> 
> May be worth some experimentation though, so I can answer this with more confidence...
It seems plausible that we might tail call in this kind of code:
```
void useit(int*, int);
void f(void (*fp)(), int n) {
  int vla[n] __attribute__((aligned(64)));
  useit(&vla[0], n);
  fp(); // suppose 'fp' lives in a fixed stack object
}
```

We probably aren't clever enough to do this today, but maybe a better assert would be to check that FrameIndex is fixed?


Repository:
  rL LLVM

https://reviews.llvm.org/D32205