[ARM] [MC] Add support for writing ARM WinEH unwind info

This includes .seh_* directives for generating it from assembly.
It is designed fairly similarly to the ARM64 handling.

For .seh_handler directives, such as
".seh_handler __C_specific_handler, @except" (which is supported
on x86_64 and aarch64 so far), the "@except" bit doesn't work in
ARM assembly, as '@' is used as a comment character (on all current
platforms).

Allow using '%' instead of '@' for this purpose. This convention
is used by GAS in similar contexts already,
e.g. [1]:

    Note on targets where the @ character is the start of a comment
    (eg ARM) then another character is used instead. For example the
    ARM port uses the % character.

In practice, this unfortunately means that all such .seh_handler
directives will need ifdefs for ARM.

Contrary to ARM64, on ARM, it's quite common that we can't evaluate
e.g. the function length at this point, due to instructions whose
length is finalized later. (Also, inline jump tables end with
a ".p2align 1".)

If unable to to evaluate the function length immediately, emit
it as an MCExpr instead. If we'd implement splitting the unwind
info for a function (which isn't implemented for ARM64 yet either),
we wouldn't know whether we need to split it though.

Avoid calling getFrameIndexOffset() on an unset
FuncInfo.UnwindHelpFrameIdx, to avoid triggering asserts in the
preexisting testcase CodeGen/ARM/Windows/wineh-basic.ll. (Once
MSVC exception handling is fully implemented, those changes
can be reverted.)

[1] https://sourceware.org/binutils/docs/as/Section.html#Section

Differential Revision: https://reviews.llvm.org/D125645

1 files changed, 8 insertions, 2 deletions

diff --git a/llvm/lib/CodeGen/AsmPrinter/WinException.cpp b/llvm/lib/CodeGen/AsmPrinter/WinException.cpp
index e00cd83..cdbd112 100644
--- a/llvm/lib/CodeGen/AsmPrinter/WinException.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/WinException.cpp
@@ -693,7 +693,12 @@ void WinException::emitCXXFrameHandler3Table(const MachineFunction *MF) {
   }
 
   int UnwindHelpOffset = 0;
-  if (Asm->MAI->usesWindowsCFI())
+  // TODO: The check for UnwindHelpFrameIdx against max() below (and the
+  // second check further below) can be removed if MS C++ unwinding is
+  // implemented for ARM, when test/CodeGen/ARM/Windows/wineh-basic.ll
+  // passes without the check.
+  if (Asm->MAI->usesWindowsCFI() &&
+      FuncInfo.UnwindHelpFrameIdx != std::numeric_limits<int>::max())
     UnwindHelpOffset =
         getFrameIndexOffset(FuncInfo.UnwindHelpFrameIdx, FuncInfo);
 
@@ -755,7 +760,8 @@ void WinException::emitCXXFrameHandler3Table(const MachineFunction *MF) {
   AddComment("IPToStateXData");
   OS.emitValue(create32bitRef(IPToStateXData), 4);
 
-  if (Asm->MAI->usesWindowsCFI()) {
+  if (Asm->MAI->usesWindowsCFI() &&
+      FuncInfo.UnwindHelpFrameIdx != std::numeric_limits<int>::max()) {
     AddComment("UnwindHelp");
     OS.emitInt32(UnwindHelpOffset);
   }