[cfe-dev] (not) initializing assembly outputs with -ftrivial-auto-var-init

[James Y Knight via cfe-dev]

Please be more specific about the problem, because your simplified example
doesn't actually show an issue. If I write this function:
int foo() {
  int retval;
  asm("# ..." : "=r"(retval));
  return retval;
}
it already does get treated as definitely writing retval, and optimizes
away the initialization (whether you explicitly initialize retval, or use
-ftrivial-auto-var-init).
Example: https://godbolt.org/z/YYBCXL

On Thu, Mar 21, 2019 at 8:35 AM Alexander Potapenko via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> Hi JF et al.,
>
> In the Linux kernel we often encounter the following pattern:
>
> type op(...) {
>   type retval;
>   inline asm(... retval ...);
>   return retval;
> }
>
> , which is used to implement low-level platform-dependent memory
> operations.
>
> Some of these operations turn out to be very hot, so we probably don't
> want to initialize |retval| given that it's always initialized in the
> assembly.
>
> However it's practically impossible to tell that a variable is being
> written to by the inline assembly, or figure out the size of that
> write.
> Perhaps we could speculatively treat every scalar output of an inline
> assembly routine as an initialized value (which is true for the Linux
> kernel, but I'm not sure about other users of inline assembly, e.g.
> video codecs).
>
> WDYT?
>
>
> --
> Alexander Potapenko
> Software Engineer
>
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