GlobalISel: Move Localizer::shouldLocalize(..) to TargetLowering

Add a new target hook for shouldLocalize so that
targets can customize the logic.

https://reviews.llvm.org/D75207

1 files changed, 62 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/TargetLoweringBase.cpp b/llvm/lib/CodeGen/TargetLoweringBase.cpp
index 95c63d0..436857d 100644
--- a/llvm/lib/CodeGen/TargetLoweringBase.cpp
+++ b/llvm/lib/CodeGen/TargetLoweringBase.cpp
@@ -18,6 +18,7 @@
 #include "llvm/ADT/Triple.h"
 #include "llvm/ADT/Twine.h"
 #include "llvm/Analysis/Loads.h"
+#include "llvm/Analysis/TargetTransformInfo.h"
 #include "llvm/CodeGen/Analysis.h"
 #include "llvm/CodeGen/ISDOpcodes.h"
 #include "llvm/CodeGen/MachineBasicBlock.h"
@@ -2072,3 +2073,64 @@ TargetLoweringBase::getAtomicMemOperandFlags(const Instruction &AI,
   Flags |= getTargetMMOFlags(AI);
   return Flags;
 }
+
+//===----------------------------------------------------------------------===//
+//  GlobalISel Hooks
+//===----------------------------------------------------------------------===//
+
+bool TargetLoweringBase::shouldLocalize(const MachineInstr &MI,
+                                        const TargetTransformInfo *TTI) const {
+  auto &MF = *MI.getMF();
+  auto &MRI = MF.getRegInfo();
+  // Assuming a spill and reload of a value has a cost of 1 instruction each,
+  // this helper function computes the maximum number of uses we should consider
+  // for remat. E.g. on arm64 global addresses take 2 insts to materialize. We
+  // break even in terms of code size when the original MI has 2 users vs
+  // choosing to potentially spill. Any more than 2 users we we have a net code
+  // size increase. This doesn't take into account register pressure though.
+  auto maxUses = [](unsigned RematCost) {
+    // A cost of 1 means remats are basically free.
+    if (RematCost == 1)
+      return UINT_MAX;
+    if (RematCost == 2)
+      return 2U;
+
+    // Remat is too expensive, only sink if there's one user.
+    if (RematCost > 2)
+      return 1U;
+    llvm_unreachable("Unexpected remat cost");
+  };
+
+  // Helper to walk through uses and terminate if we've reached a limit. Saves
+  // us spending time traversing uses if all we want to know is if it's >= min.
+  auto isUsesAtMost = [&](unsigned Reg, unsigned MaxUses) {
+    unsigned NumUses = 0;
+    auto UI = MRI.use_instr_nodbg_begin(Reg), UE = MRI.use_instr_nodbg_end();
+    for (; UI != UE && NumUses < MaxUses; ++UI) {
+      NumUses++;
+    }
+    // If we haven't reached the end yet then there are more than MaxUses users.
+    return UI == UE;
+  };
+
+  switch (MI.getOpcode()) {
+  default:
+    return false;
+  // Constants-like instructions should be close to their users.
+  // We don't want long live-ranges for them.
+  case TargetOpcode::G_CONSTANT:
+  case TargetOpcode::G_FCONSTANT:
+  case TargetOpcode::G_FRAME_INDEX:
+  case TargetOpcode::G_INTTOPTR:
+    return true;
+  case TargetOpcode::G_GLOBAL_VALUE: {
+    unsigned RematCost = TTI->getGISelRematGlobalCost();
+    Register Reg = MI.getOperand(0).getReg();
+    unsigned MaxUses = maxUses(RematCost);
+    if (MaxUses == UINT_MAX)
+      return true; // Remats are "free" so always localize.
+    bool B = isUsesAtMost(Reg, MaxUses);
+    return B;
+  }
+  }
+}