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[Windows SEH] Fix the frame-ptr of a nested-filter within a _finally

Authored by tentzen on Jul 12 2020, 1:37 AM.

Description

[Windows SEH] Fix the frame-ptr of a nested-filter within a _finally

This change fixed a SEH bug (exposed by test58 & test61 in MSVC test xcpt4u.c);
when an Except-filter is located inside a finally, the frame-pointer generated today
via intrinsic @llvm.eh.recoverfp is the frame-pointer of the immediate
parent _finally, not the frame-ptr of outermost host function.

The fix is to retrieve the Establisher's frame-pointer that was previously saved in
parent's frame.
The prolog of a filter inside a _finally should be like code below:

%0 = call i8* @llvm.eh.recoverfp(i8* bitcast (@"?fin$0@0@main@@"), i8*%frame_pointer)
%1 = call i8* @llvm.localrecover(i8* bitcast (@"?fin$0@0@main@@"), i8*%0, i32 0)
%2 = bitcast i8* %1 to i8
%3 = load i8*, i8
%2, align 8

Differential Revision: https://reviews.llvm.org/D77982