[llvm-dev] allocsize: change from 3.9 to 4.0

[Johan Engelen via llvm-dev]

Hi all,
  I added support for the allocsize function attribute to our compiler
(LDC), thinking that that would enable removal of function calls when the
allocated memory is not used.

For example:
```
declare i8* @my_malloc(i32) allocsize(0)

define void @test_malloc() {
  %1 = call i8* @my_malloc(i32 100)
  ret void
}
```
I thought the my_alloc call in test_malloc would be removed, but `opt -O3`
doesn't do that (LLVM 4.0 and trunk). However, LLVM3.9's `opt` _does_
remove the call.

I can't find out why this was changed.
(if the call is to "malloc", it is removed because LLVM recognizes the
function name)

Thanks for the explanation,
Kind regards,
  Johan
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