AArch64: refactor frame handling to use movz/movk for overlarge offsets.

In the near future litpools will be in a different section, which means that
any access to them is at least two instructions. This makes the case for a
movz/movk pair (if total offset <= 32-bits) even more compelling.

llvm-svn: 175257

1 files changed, 29 insertions, 12 deletions

diff --git a/llvm/lib/Target/AArch64/AArch64InstrInfo.cpp b/llvm/lib/Target/AArch64/AArch64InstrInfo.cpp
index 94b342952a8..9a7504afd9f 100644
--- a/llvm/lib/Target/AArch64/AArch64InstrInfo.cpp
+++ b/llvm/lib/Target/AArch64/AArch64InstrInfo.cpp
@@ -623,18 +623,35 @@ void llvm::emitRegUpdate(MachineBasicBlock &MBB,
   else if (abs(NumBytes) & ~0xffffff) {
     // Generically, we have to materialize the offset into a temporary register
     // and subtract it. There are a couple of ways this could be done, for now
-    // we'll go for a literal-pool load.
-    MachineFunction &MF = *MBB.getParent();
-    MachineConstantPool *MCP = MF.getConstantPool();
-    const Constant *C
-      = ConstantInt::get(Type::getInt64Ty(MF.getFunction()->getContext()),
-                         abs(NumBytes));
-    unsigned CPI = MCP->getConstantPoolIndex(C, 8);
-
-    // LDR xTMP, .LITPOOL
-    BuildMI(MBB, MBBI, dl, TII.get(AArch64::LDRx_lit), ScratchReg)
-      .addConstantPoolIndex(CPI)
-      .setMIFlag(MIFlags);
+    // we'll use a movz/movk or movn/movk sequence.
+    uint64_t Bits = static_cast<uint64_t>(abs(NumBytes));
+    BuildMI(MBB, MBBI, dl, TII.get(AArch64::MOVZxii), ScratchReg)
+      .addImm(0xffff & Bits).addImm(0)
+      .setMIFlags(MIFlags);
+
+    Bits >>= 16;
+    if (Bits & 0xffff) {
+      BuildMI(MBB, MBBI, dl, TII.get(AArch64::MOVKxii), ScratchReg)
+        .addReg(ScratchReg)
+        .addImm(0xffff & Bits).addImm(1)
+        .setMIFlags(MIFlags);
+    }
+
+    Bits >>= 16;
+    if (Bits & 0xffff) {
+      BuildMI(MBB, MBBI, dl, TII.get(AArch64::MOVKxii), ScratchReg)
+        .addReg(ScratchReg)
+        .addImm(0xffff & Bits).addImm(2)
+        .setMIFlags(MIFlags);
+    }
+
+    Bits >>= 16;
+    if (Bits & 0xffff) {
+      BuildMI(MBB, MBBI, dl, TII.get(AArch64::MOVKxii), ScratchReg)
+        .addReg(ScratchReg)
+        .addImm(0xffff & Bits).addImm(3)
+        .setMIFlags(MIFlags);
+    }
 
     // ADD DST, SRC, xTMP (, lsl #0)
     unsigned AddOp = NumBytes > 0 ? AArch64::ADDxxx_uxtx : AArch64::SUBxxx_uxtx;