[PATCH] D34367: CodeGen: Fix address space of indirect function argument

[Yaxun Liu via Phabricator via cfe-commits]

yaxunl added inline comments.


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Comment at: lib/CodeGen/CGCall.h:211
+      push_back(CallArg(rvalue, type, needscopy, AS));
     }
 
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rjmccall wrote:
> Ah, I think I understand what's going on here now.  "indirect byval" arguments include an implicit copy to the stack, and the code is trying to avoid emitting an extra copy to a temporary in the frontend.  It does this by recognizing loads from aggregate l-values and just adding the l-value to the call-argument list as a "needs copy" argument.  Call-argument preparation then recognizes that the argument is being passed indirect byval and, under certain conditions, just uses the l-value address as the IR argument.
> 
> Instead of creeping more and more information from the LValue into the CallArg, I think there are two reasonable alternatives:
> 
> - Suppress this optimization in EmitCallArg when the LValue isn't in the alloca address space.  EmitCallArg already suppresses it when the argument is under-aligned.
> 
> - Find a way to actually store an LValue in a CallArg.  This has the advantage that we can assert if someone tries to access it as an RValue, which is good because any attempt to use a needs-copy argument as if it were a normal argument is a lurking bug.
Sorry for the delay. I updated the patch with by the second approach. I did not add a member of LValue type to CallArg but added a member of Expr* type since sometimes LValue is not available. The Expr is the l-value expression from which the r-value is derived.


https://reviews.llvm.org/D34367