LLVM back end

Simon Peyton-Jones simonpj at microsoft.com
Thu Dec 21 03:14:45 EST 2006

How about
* concurrency (the ability to have zillions of little stacks,
        with stack overflow checks and growing the stck on overflow)?
* exception handling (the ability to crawl over the stack
        looking for exception catch frames)?
* garbage collection (the ability to find pointers in the stack)


| -----Original Message-----
| From: glasgow-haskell-users-bounces at haskell.org [mailto:glasgow-
| haskell-users-bounces at haskell.org] On Behalf Of Wolfgang Thaller
| Sent: 20 December 2006 23:55
| To: ttmrichter at gmail.com
| Cc: GHC Users; Simon Marlow
| Subject: Re: LLVM back end
| On 20-Dec-06, at 10:10 AM, Michael T. Richter wrote:
| > Well, I'm almost entirely ignorant of LLVM and of Haskell --
| > especially the internals of both.  That makes me ideally suited for
| > this project since I'm not aware that it's impossible.
| I'm afraid LLVM currently lacks some features that are required for
| efficient GHC code generation, specifically:
| a) Global Register Variables
| b) The ability to put data next to code
| c) (maybe, I'm not sure) Tailcalls
| ad a)
| We need to keep some things in global registers for speed reasons
| (depending on the number of available registers); among other things,
| the haskell stack pointer and the heap pointer. Using regular global
| variables instead would be a lot slower.
| ad b)
| With (almost) every chunk of code, we want to associate a smal chunk
| of data (the "info table") which contains information used by the
| garbage collector and other parts of the run time system. We want to
| use only one pointer to point to both of those things, and we don't
| want to waste time with any additional indirections, so we make the
| pointer point between the data chunk and the code chunk.
| ad c)
| While they are supported in theory, I couldn't get LLVM to generate
| any tailcalls. Maybe I've done something wrong, maybe they're not
| really implemented yet.
| So I guess step one would be to start negotiating about those things
| with the LLVM people.
| Cheers,
| Wolfgang
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