[PATCH] AArch64: Make AArch64A57FPLoadBalancing output stable.

[James Molloy]

Hi Geoff,

Thanks for fixing this! It looks ok as-is, but wouldn't switching simply
from std::sort to std::stable_sort solve the problem easier? Stable_sort is
used for this reason in other areas of the compiler.

Cheers,

James
On Fri, 30 Jan 2015 at 03:51, Geoff Berry <gberry at codeaurora.org> wrote:

> Hi jmolloy, mcrosier, apazos,
>
> Add tie breaker to colorChainSet() sort so that processing order doesn't
> depend on std::set order, which depends on pointer order, which is
> unstable from run to run.
>
> REPOSITORY
>   rL LLVM
>
> http://reviews.llvm.org/D7265
>
> Files:
>   lib/Target/AArch64/AArch64A57FPLoadBalancing.cpp
>
> Index: lib/Target/AArch64/AArch64A57FPLoadBalancing.cpp
> ===================================================================
> --- lib/Target/AArch64/AArch64A57FPLoadBalancing.cpp
> +++ lib/Target/AArch64/AArch64A57FPLoadBalancing.cpp
> @@ -259,7 +259,7 @@
>    }
>
>    /// Return true if this chain starts before Other.
> -  bool startsBefore(Chain *Other) {
> +  bool startsBefore(const Chain *Other) const {
>      return StartInstIdx < Other->StartInstIdx;
>    }
>
> @@ -431,10 +431,17 @@
>    // chains that we cannot change before we look at those we can,
>    // so the parity counter is updated and we know what color we should
>    // change them to!
> +  // Final tie-break with instruction order so pass output is stable
> (i.e. not
> +  // dependent on malloc'd pointer values).
>    std::sort(GV.begin(), GV.end(), [](const Chain *G1, const Chain *G2) {
>        if (G1->size() != G2->size())
>          return G1->size() > G2->size();
> -      return G1->requiresFixup() > G2->requiresFixup();
> +      if (G1->requiresFixup() != G2->requiresFixup())
> +        return G1->requiresFixup() > G2->requiresFixup();
> +      // Make sure startsBefore() produces a stable final order.
> +      assert((G1 == G2 || (G1->startsBefore(G2) ^ G2->startsBefore(G1)))
> &&
> +             "Starts before not total order!");
> +      return G1->startsBefore(G2);
>      });
>
>    Color PreferredColor = Parity < 0 ? Color::Even : Color::Odd;
>
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