[M3devel] higher level M3CG modeling of jmpbuf/setjmp/longjmp or exception handling?

Jay K jay.krell at cornell.edu
Thu Jul 30 06:36:34 CEST 2015

On the last point -- I'm willing to generate C++ instead of C. Does that help?I think it probably does.

On Win32 I could still generate C. And, not important, but Ultrix, OSF/1, and OpenVMS Ithink all support what Win32 support -- not a coincidence..

One thing I'm missing is I don't know really know how exceptions are lowered currently.Like, what does the runtime data look like for the "filtering".I should know better..

I think we can also essentially claim all targets have a stack walker.One way or another -- libunwind, NT RtlUnwindEx, etc.

 - Jay

Subject: Re: [M3devel] higher level M3CG modeling of jmpbuf/setjmp/longjmp or exception handling?
From: hosking at purdue.edu
Date: Thu, 30 Jul 2015 14:04:14 +1000
CC: m3devel at elegosoft.com
To: jay.krell at cornell.edu

I agree that lifting the abstraction level for exceptions in the front-end would be a good idea.It would allow us to more effectively make use of specialized target exception handling, e.g., from LLVM.That suggests we at least lift to the LLVM level of abstraction, but I don’t know how easy that will be to generate from for the C backend.
On Jul 30, 2015, at 10:38 AM, Jay K <jay.krell at cornell.edu> wrote:I don't have one precise proposal, but somehow,jmpbuf/setjmp/longjmp should be represented in a higher level in m3cgor maybe "even higher level", exception handling.

There are multiple optimization opportunties and multipleopportunities to move/remove target-dependent code.

In particular m3middle/m3front should not know jmpbuf size.Generated C should not have jmpbuf sizes embedded in it.

gcc has a "builtin" jmpbuf/setjmp/longjmp notion, for use by exceptionhandling, that llvm replicates, that is optimized, like to notsave/restore unused registers.

Or, yes, there is my alloca proposal, which doesn't solve as much, but does help.

In particular, the C backend just wants to know something is a "jmpbuf"so it can #include <setjmp.h> in the generated code and produce ajmpbuf instead of an array of bytes and call setjmp/longjmp or _setjmp/_longjmp.

Or even better, a C++ backend wants to somehow module Modula-3 exceptionsas C++ exceptions.

Or, possibly, the Win32 C backend could model Modula-3 exceptions using"structured exception handling" -- advantage over C++ exceptions is portabilityto kernel mode.

Thoughts? - Jay

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