[PATCH] AArch64: Make AArch64A57FPLoadBalancing output stable.

[Geoff Berry]

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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