On 12/01/2016 06:22 AM, Richard Biener wrote:
Well after removing DECL_BY_REFERENCE, verify_gimple still fails but

tail-padding1.C:13:1: error: RESULT_DECL should be read only when
while verifying SSA_NAME nrvo_4 in statement
# .MEM_3 = VDEF <.MEM_1(D)>
    nrvo_4 = __builtin_memset (nrvo_2(D), 0, 8);
tail-padding1.C:13:1: internal compiler error: verify_ssa failed

Hmm, ok.  Not sure why we enforce this.
I don't know either. But I would start by looking at tree-nrv.c since it looks (based on the variable names) that the named-value-return optimization kicked in.

Note that in the end this patch looks fishy -- iff we really need
the LHS on the assignment for correctness if we have the tailcall
flag set then what guarantees that later passes do not remove it
again?  So anybody removing a LHS would need to unset the tailcall flag?

Saying again that I don't know enough about the RTL part of tailcall
The LHS on the assignment makes it easier to identify when a tail call is possible. It's not needed for correctness. Not having the LHS on the assignment just means we won't get an optimized tail call.

Under what circumstances would the LHS possibly be removed? We know the return statement references the LHS, so it's not going to be something that DCE will do.


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