[LLVMdev] RFC: Better alternative to llvm.frameallocate for use in Windows EH

[Reid Kleckner]

I realized that WinEH preparation can probably be a lot less invasive than
it is currently.

Initially, when I was thinking about recovering the address of an object in
a parent stack frame, I thought about it in terms of "let's allocate
something at a fixed offset from ebp to keep things simple". That line of
thinking suggested that we needed this thing to be fundamentally different
from a normal alloca. I was going to make sure it was allocated early in
the prologue, for example.

However, this never happened, and having a fixed offset isn't very simple.
I ended up ditching the fixed offset and using assembly label assignments
to communicate frame index offsets between parent functions and outlined
subfunctions. This technique easily generalizes to support referencing an
arbitrary number of allocations in the parent frame, and I think we should
go ahead and do that.

The current approach has downsides that we take a bunch of vanilla allocas
and SSA values in the parent function and mash them into a single
allocation, and replace the accesses with GEPs of an intrinsic result. This
is a lot of funky looking IR for something that should be really simple. We
also already have good isel for accessing allocas, and we lose that when we
switch to an intrinsic.

So instead, let's go back to using normal allocas and "blessing" each of
them as escaped allocations that can be referenced from EH helpers. Here's
what it would look like:

define i32 @parent() {
  %a = alloca i32
  %b = alloca i32
  call void (...)* @llvm.frameescape(i32* %a, i32* %b)
  %fp = call i8* @llvm.frameaddress(i32 0)
  call void @helper_func(i8* %fp)
  %a_val = load i32, i32* %a
  %b_val = load i32, i32* %b
  %r = add i32 %a_val, %b_val
  ret i32 %r
}

define void @helper_func(i8* %fp) {
  %a.i8 = call i8* @llvm.framerecover(i8* bitcast (i32 ()* @parent to i8*),
i8* %fp, i32 0)
  %b.i8 = call i8* @llvm.framerecover(i8* bitcast (i32 ()* @parent to i8*),
i8* %fp, i32 1)
  %a = bitcast i8* %a.i8 to i32*
  %b = bitcast i8* %b.i8 to i32*
  store i32 1, i32* %a
  store i32 2, i32* %b
  ret void
}

declare i8* @llvm.frameaddress(i32)
declare i8* @llvm.framerecover(i8*, i8*, i32)
declare void @llvm.frameescape(...)

In this example, 'helper_func' is able to access the frame of 'parent'.
'parent' should return 3.

With this, we can outline landingpads without disturbing the parent
function nearly as much. It should help -O0 codegen time, as we don't need
as much replacement. It just seems nicer.

We still have the same potential codegen problems in the outlined helpers
that we did in the parent function, but recall that the outlined functions
only get called when an exception is thrown, ie practically never. I'm OK
if we have to hack up the IR for helper function by sinking lots of
@llvm.framerecover calls into all the BBs that use the original alloca. I'm
OK if the isel is slightly worse than it could be.

Thoughts?
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