[PATCH] D71499: Add builtins for aligning and checking alignment of pointers and integers

[David Chisnall via Phabricator via cfe-commits]

theraven added inline comments.


================
Comment at: clang/lib/CodeGen/CGBuiltin.cpp:14323
+    Result = Builder.CreateIntrinsic(
+        Intrinsic::ptrmask, {Args.SrcType, SrcForMask->getType(), Args.IntType},
+        {SrcForMask, NegatedMask}, nullptr, "aligned_result");
----------------
fhahn wrote:
> lebedev.ri wrote:
> > arichardson wrote:
> > > lebedev.ri wrote:
> > > > Is sufficient amount of passes, analyses know about this intrinsic?
> > > Good question. In the simple test cases that I looked at the code generation was equivalent. 
> > > 
> > > In our fork we still use ptrtoint+inttoptr since I implemented them before the new intrinsic existed. But since the ptrmask instrinsic exists now I thought I'd use it for upstreaming.
> > > I'll investigate if this results in worse codegen for more complex uses.
> > > 
> > (TLDR: before producing it in more cases in clang, i think it should be first ensured
> > that everything in middle-end is fully aware of said intrinsic. (i.e. using it vs it's
> > exploded form results in no differences in final assembly on a sufficient test coverage))
> > I'll investigate if this results in worse codegen for more complex uses.
> 
> The findings would be interesting indeed. 
> 
> One thing to watch out for is that ptrmask should give better alias analysis results than ptrtoint/inttoptr. Also, IIRC some instcombine transformations for ptrtoint/inttoptr are not strictly valid according to the LangRef. Not sure if that changed yet.
There is currently an open review on the semantics of inttoptr / ptrtoint.  The current LangRef underspecifies it.  On most architectures, both are bitcasts.  On CHERI (including ARM's Morello system), it is not safe to round trip a pointer via an integer at the IR or machine-code level (in C, `[u]intptr_t` is represented as `i8*` in the IR).  A few architectures do some arithmetic to cast between pointer and integer.  The mid-level optimisers are slowly getting better at avoiding the patterns that rely on the underspecified behaviour, but removing this need for them would be a benefit.


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  https://reviews.llvm.org/D71499/new/

https://reviews.llvm.org/D71499