[AArch64] FPLoadBalancing: move ownership of the chain to its current accumulator register

and forget about the previously used accumulator.

Coming up with a simple testcase is not easy, as this highly depends on
what the register allocator is doing: this issue showed up while working
with the PBQP allocator, which produced a different allocation scheme.
A testcase would need to come up with chain starting in D[0-7], then
moving to D[8-15], followed by a call to a function whose regmask
clobbers the starting accumulator in D[0-7], then another use of the chain.

Fixed some formatting, added some invariant checks while there.

llvm-svn: 216721

1 files changed, 14 insertions, 5 deletions

diff --git a/llvm/lib/Target/AArch64/AArch64A57FPLoadBalancing.cpp b/llvm/lib/Target/AArch64/AArch64A57FPLoadBalancing.cpp
index caf4ca3e3db..c3047c50e35 100644
--- a/llvm/lib/Target/AArch64/AArch64A57FPLoadBalancing.cpp
+++ b/llvm/lib/Target/AArch64/AArch64A57FPLoadBalancing.cpp
@@ -193,10 +193,10 @@ public:
   /// instruction can be more tricky.
   Color LastColor;
 
-  Chain(MachineInstr *MI, unsigned Idx, Color C) :
-  StartInst(MI), LastInst(MI), KillInst(NULL),
-  StartInstIdx(Idx), LastInstIdx(Idx), KillInstIdx(0),
-  LastColor(C) {
+  Chain(MachineInstr *MI, unsigned Idx, Color C)
+      : StartInst(MI), LastInst(MI), KillInst(nullptr),
+        StartInstIdx(Idx), LastInstIdx(Idx), KillInstIdx(0),
+        LastColor(C) {
     Insts.insert(MI);
   }
 
@@ -206,6 +206,9 @@ public:
     LastInst = MI;
     LastInstIdx = Idx;
     LastColor = C;
+    assert((KillInstIdx == 0 || LastInstIdx < KillInstIdx) &&
+           "Chain: broken invariant. A Chain can only be killed after its last "
+           "def");
 
     Insts.insert(MI);
   }
@@ -224,6 +227,9 @@ public:
     KillInst = MI;
     KillInstIdx = Idx;
     KillIsImmutable = Immutable;
+    assert((KillInstIdx == 0 || LastInstIdx < KillInstIdx) &&
+           "Chain: broken invariant. A Chain can only be killed after its last "
+           "def");
   }
 
   /// Return the first instruction in the chain.
@@ -626,7 +632,10 @@ scanInstruction(MachineInstr *MI, unsigned Idx,
         DEBUG(dbgs() << "Instruction was successfully added to chain.\n");
         ActiveChains[AccumReg]->add(MI, Idx, getColor(DestReg));
         // Handle cases where the destination is not the same as the accumulator.
-        ActiveChains[DestReg] = ActiveChains[AccumReg];
+        if (DestReg != AccumReg) {
+          ActiveChains[DestReg] = ActiveChains[AccumReg];
+          ActiveChains.erase(AccumReg);
+        }
         return;
       }