[LLVMdev] Advice on MachineMoves and SEH

Charles Davis cdavis at mymail.mines.edu
Wed Jun 1 21:27:19 CDT 2011


Hi,

If you've been following llvm-commits, you might know that I've been
working on implementing support for SEH--specifically, the Win64 variant
of it--in LLVM.

I know a lot of you couldn't care less about this, but I'd really
appreciate some advice about this. I'm almost to the point where it's
possible to use GCC-style exceptions under Win64, but I've hit a small
roadblock.

The problem is that I need information about what happens to the stack
in the prologue. I know that information is stored in MachineMove
objects in the MachineModuleInfo, but this information seems to be
specific to DWARF CFI.

Windows' scheme for storing information about the call frame differs
somewhat from DWARF CFI. For one thing, the distinction between an x86
PUSH and MOV onto the stack is very important. Windows (and compatibles)
will actually execute the prologue in reverse, instead of just restoring
the registers' state. As a result, Windows needs to know that a register
was PUSHed onto the stack and not simply MOVed.

Another difference is in how offsets on the stack are recorded. In DWARF
CFI, they're offsets from the CFA. But in Win64, they are offsets from
the stack pointer (%rsp on x86). Even worse, if there's a frame pointer,
the offset is from %rsp *when the frame pointer was established*.

Because of all this, it's hard to reconstruct the SEH information from
the MachineMove array. I have thought about adding a new array specific
to SEH information, but I'm not sure how you guys would feel about that.
Any ideas on how to solve this problem?

Chip


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