[PATCH] D48589: [WIP] [CodeGen] Allow specifying Extend to CoerceAndExpand

[John McCall via Phabricator via cfe-commits]

rjmccall added inline comments.


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Comment at: include/clang/CodeGen/CGFunctionInfo.h:90
   union {
-    unsigned DirectOffset;     // isDirect() || isExtend()
-    unsigned IndirectAlign;    // isIndirect()
-    unsigned AllocaFieldIndex; // isInAlloca()
+    llvm::StructType *ExtendSeq; // isCoerceAndExpand()
+    unsigned DirectOffset;       // isDirect() || isExtend()
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rogfer01 wrote:
> rjmccall wrote:
> > Hmm.  I understand the need to use something uniqued here, but I think it would probably be more natural to at least use a `llvm::ConstantDataArray` (which could be null if there are no interesting bits) instead of encoding the information into the types of a struct type.  That would also make it easy to generalize the elements to an arbitrary flags type.
> > 
> > Also, on 64-bit targets this will increase the size of an `ABIArgInfo` to four pointers from three.  That's fine to the extent that we work with independent `ABIArgInfo`s, but I'm getting a little annoyed at the storage overhead of the array of `ABIArgInfo`s in `CGFunctionInfo` given that, in the overwhelmingly common case, an `ABIArgInfo` is no more than a kind and maybe a few of these flags.  Maybe there's some reasonable way to optimize the storage of an `ABIArgInfo` in a `CGFunctionInfo` so that we only need the extra storage in less-common cases?  Like extracting out a base class that's the Kind+Flags and making the main array be an array of those + an optional index into a second trailing array of full `ABIArgInfo`s.
> > 
> > I might be overthinking this, though.
> Oh, of course, using a value rather than a type is more sensible. I will change this.
> 
> The idea of having a lighter `ABIArgInfo` and an ancillary extra array for it seems sensible. I guess it may depend on how early/late we know the precise kind of `ABIArgInfo` we need to represent. I haven't looked at this yet, I'll investigate.
`CGFunctionInfo` should be totally immutable and just ultimately constructed with an array of `ABIArgInfo`, which seems like a reasonable place to inject some representation tricks.


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Comment at: include/clang/CodeGen/CGFunctionInfo.h:475
 
+static_assert(std::is_trivially_copyable<ABIArgInfo>::value,
+    "ABIArgInfo must be trivially copyable as it is embedded as trailing "
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rogfer01 wrote:
> rogfer01 wrote:
> > I think this is the right trait here. I spent too much time debugging this :)
> But today I realised that GCC 4.9 does not implement this so it will have to go away :(
You could gate it on the compiler if you want, maybe extracting a `HAS_IS_TRIVIALLY_COPYABLE` test macro in `llvm/Support/type_traits.h` out of the implementation of `llvm::isPodLike`.


https://reviews.llvm.org/D48589