[GlobalISel][Utils] Add a getConstantVRegVal variant that looks through instrs

getConstantVRegVal used to only look for G_CONSTANT when looking at
unboxing the value of a vreg. However, constants are sometimes not
directly used and are hidden behind trunc, s|zext or copy chain of
computation.

In particular this may be introduced by the legalization process that
doesn't want to simplify these patterns because it can lead to infine
loop when legalizing a constant.

To circumvent that problem, add a new variant of getConstantVRegVal,
named getConstantVRegValWithLookThrough, that allow to look through
extensions.

Differential Revision: https://reviews.llvm.org/D59227

llvm-svn: 356116

2 files changed, 60 insertions, 10 deletions

diff --git a/llvm/lib/CodeGen/GlobalISel/InstructionSelector.cpp b/llvm/lib/CodeGen/GlobalISel/InstructionSelector.cpp
index d8cd71f64b9..a449e9dec41 100644
--- a/llvm/lib/CodeGen/GlobalISel/InstructionSelector.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/InstructionSelector.cpp
@@ -49,8 +49,8 @@ bool InstructionSelector::isOperandImmEqual(
     const MachineOperand &MO, int64_t Value,
     const MachineRegisterInfo &MRI) const {
   if (MO.isReg() && MO.getReg())
-    if (auto VRegVal = getConstantVRegVal(MO.getReg(), MRI))
-      return *VRegVal == Value;
+    if (auto VRegVal = getConstantVRegValWithLookThrough(MO.getReg(), MRI))
+      return VRegVal->Value == Value;
   return false;
 }
 
diff --git a/llvm/lib/CodeGen/GlobalISel/Utils.cpp b/llvm/lib/CodeGen/GlobalISel/Utils.cpp
index 237d892c7f7..d65328be39c 100644
--- a/llvm/lib/CodeGen/GlobalISel/Utils.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/Utils.cpp
@@ -183,18 +183,68 @@ void llvm::reportGISelFailure(MachineFunction &MF, const TargetPassConfig &TPC,
 
 Optional<int64_t> llvm::getConstantVRegVal(unsigned VReg,
                                            const MachineRegisterInfo &MRI) {
-  MachineInstr *MI = MRI.getVRegDef(VReg);
-  if (MI->getOpcode() != TargetOpcode::G_CONSTANT)
+  Optional<ValueAndVReg> ValAndVReg =
+      getConstantVRegValWithLookThrough(VReg, MRI, /*LookThroughInstrs*/ false);
+  assert((!ValAndVReg || ValAndVReg->VReg == VReg) &&
+         "Value found while looking through instrs");
+  if (!ValAndVReg)
+    return None;
+  return ValAndVReg->Value;
+}
+
+Optional<ValueAndVReg> llvm::getConstantVRegValWithLookThrough(
+    unsigned VReg, const MachineRegisterInfo &MRI, bool LookThroughInstrs) {
+  SmallVector<std::pair<unsigned, unsigned>, 4> SeenOpcodes;
+  MachineInstr *MI;
+  while ((MI = MRI.getVRegDef(VReg)) &&
+         MI->getOpcode() != TargetOpcode::G_CONSTANT && LookThroughInstrs) {
+    switch (MI->getOpcode()) {
+    case TargetOpcode::G_TRUNC:
+    case TargetOpcode::G_SEXT:
+    case TargetOpcode::G_ZEXT:
+      SeenOpcodes.push_back(std::make_pair(
+          MI->getOpcode(),
+          MRI.getType(MI->getOperand(0).getReg()).getSizeInBits()));
+      VReg = MI->getOperand(1).getReg();
+      break;
+    case TargetOpcode::COPY:
+      VReg = MI->getOperand(1).getReg();
+      if (TargetRegisterInfo::isPhysicalRegister(VReg))
+        return None;
+      break;
+    default:
+      return None;
+    }
+  }
+  if (!MI || MI->getOpcode() != TargetOpcode::G_CONSTANT ||
+      (!MI->getOperand(1).isImm() && !MI->getOperand(1).isCImm()))
     return None;
 
-  if (MI->getOperand(1).isImm())
-    return MI->getOperand(1).getImm();
+  const MachineOperand &CstVal = MI->getOperand(1);
+  unsigned BitWidth = MRI.getType(MI->getOperand(0).getReg()).getSizeInBits();
+  APInt Val = CstVal.isImm() ? APInt(BitWidth, CstVal.getImm())
+                             : CstVal.getCImm()->getValue();
+  assert(Val.getBitWidth() == BitWidth &&
+         "Value bitwidth doesn't match definition type");
+  while (!SeenOpcodes.empty()) {
+    std::pair<unsigned, unsigned> OpcodeAndSize = SeenOpcodes.pop_back_val();
+    switch (OpcodeAndSize.first) {
+    case TargetOpcode::G_TRUNC:
+      Val = Val.trunc(OpcodeAndSize.second);
+      break;
+    case TargetOpcode::G_SEXT:
+      Val = Val.sext(OpcodeAndSize.second);
+      break;
+    case TargetOpcode::G_ZEXT:
+      Val = Val.zext(OpcodeAndSize.second);
+      break;
+    }
+  }
 
-  if (MI->getOperand(1).isCImm() &&
-      MI->getOperand(1).getCImm()->getBitWidth() <= 64)
-    return MI->getOperand(1).getCImm()->getSExtValue();
+  if (Val.getBitWidth() > 64)
+    return None;
 
-  return None;
+  return ValueAndVReg{Val.getSExtValue(), VReg};
 }
 
 const llvm::ConstantFP* llvm::getConstantFPVRegVal(unsigned VReg,