EXTERN PROC isn't really well documented in MSVC, so after poking around
it seems as if it's just a regular extern symbol.
Interestingly enough, under MSVC the following is allowed:
extern foo:proc
mov eax, foo
MSVC will output:
mov eax, 0
while llvm-ml will currently output:
mov eax, dword ptr [foo]
(since foo is an extern)
Arguably, llvm-ml's output makes more sense, even though it's
inconsistent with MSVC ml. However, since moving an extern proc symbol
to a register doesn't really make sense in the first place, we'll treat
it as undefined behavior for now.
Reviewed By: epastor
Differential Revision: https://reviews.llvm.org/D125582
In MASM, if a QWORD symbol is passed to a jmp or call instruction in
64-bit mode or a DWORD or WORD symbol is passed in 32-bit mode, then
MSVC's assembler recognizes that as an indirect call. Additionally, if
the operand is qualified as a ptr, then that should also be an indirect
call.
Furthermore, in 64-bit mode, such operands are implicitly rip-relative
(in fact, MSVC's assembler ml64.exe does not allow explicitly specifying
rip as a base register.)
To keep this patch managable, this patch does not include:
* error messages for wrong operand types (e.g. passing a QWORD in 32-bit
mode)
* resolving indirect calls if the symbol is declared after it's first
use (llvm-ml currently only runs a single pass).
* imlementing the extern keyword (required to resolve
https://crbug.com/762167.)
This patch is likely missing a bunch of edge cases, so please do point
them out in the review.
Reviewed By: epastor, hans, MaskRay
Committed By: epastor (on behalf of ayzhao)
Differential Revision: https://reviews.llvm.org/D124413
Alan Zhao