[llvm-dev] llvm optimizer turning musttail into tail

[Mehdi Amini via llvm-dev]

I don’t think dropping from “must tail” to “tail” is a legal thing to do in the IR, isn’t there a bug in InstCombine here?

— 
Mehdi

> On Nov 27, 2016, at 12:57 PM, David Majnemer via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> Try using the "thunk" attribute: http://llvm.org/docs/LangRef.html#function-attributes <http://llvm.org/docs/LangRef.html#function-attributes>
> 
> InstCombine has logic to avoid getting rid of the cast in such situations: https://github.com/llvm-mirror/llvm/blob/master/lib/Transforms/InstCombine/InstCombineCalls.cpp#L2821 <https://github.com/llvm-mirror/llvm/blob/master/lib/Transforms/InstCombine/InstCombineCalls.cpp#L2821>
> 
> 
> 
> On Sun, Nov 27, 2016 at 7:40 AM, Carlo Kok <ck at remobjects.com <mailto:ck at remobjects.com>> wrote:
> Inst combine yes. However it shouldn't drop this bit cast in the first place, since the last parameter isn't available. (It only works because of this must tail)
> 
> 
> On 27 November 2016 01:37:43 CET, Sean Silva <chisophugis at gmail.com <mailto:chisophugis at gmail.com>> wrote:
> r287955 seems like it might be related.
> 
> -- Sean Silva
> 
> On Sat, Nov 26, 2016 at 4:06 PM, Sean Silva <chisophugis at gmail.com <mailto:chisophugis at gmail.com>> wrote:
> This sounds buggy to me. What pass is doing this?
> 
> -- Sean Silva
> 
> On Thu, Nov 24, 2016 at 5:39 AM, Carlo Kok via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
> 
> I've got some calls like:
> 
> musttail call void bitcast (i32 (i32, i8*, %Type*)* @MyMethod to void (i32, i8*)*)(i32 %0, i8* %1)
> ret void
> 
> Into something like:
> %8 = tail call i32 @MyMethod(i32 %0, i8* %1, %Type* null)
> ret void
> 
> I realize I'm losing a parameter there, but this is an interface jump trick I use and relies on the end code being a 'jmp' (x86). I realize i can probably use noopt & noinline to do this trick, but I do want llvm to optimize and inline calls if it can prove it always calls a given method. Any way i can do this?
> 
> -- 
> Carlo Kok
> RemObjects Software
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> 
> 
> -- 
> Carlo Kok
> RemObjects Software
> 
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