1 files changed, 16 insertions, 0 deletions
diff --git a/llvm/lib/Target/ARM/ARMConstantIslandPass.cpp b/llvm/lib/Target/ARM/ARMConstantIslandPass.cpp
index 2c4738d3cb7..5e97c4cb35e 100644
--- a/llvm/lib/Target/ARM/ARMConstantIslandPass.cpp
+++ b/llvm/lib/Target/ARM/ARMConstantIslandPass.cpp
@@ -1420,6 +1420,22 @@ void ARMConstantIslands::createNewWater(unsigned CPUserIndex,
       MI = LastIT;
   }
 
+  // Avoid splitting a MOVW+MOVT pair with a relocation on Windows.
+  // On Windows, this instruction pair is covered by one single
+  // IMAGE_REL_ARM_MOV32T relocation which covers both instructions. If a
+  // constant island is injected inbetween them, the relocation will clobber
+  // the instruction and fail to update the MOVT instruction.
+  // (These instructions are bundled up until right before the ConstantIslands
+  // pass.)
+  if (STI->isTargetWindows() && isThumb && MI->getOpcode() == ARM::t2MOVTi16 &&
+      (MI->getOperand(2).getTargetFlags() & ARMII::MO_OPTION_MASK) ==
+          ARMII::MO_HI16) {
+    --MI;
+    assert(MI->getOpcode() == ARM::t2MOVi16 &&
+           (MI->getOperand(1).getTargetFlags() & ARMII::MO_OPTION_MASK) ==
+               ARMII::MO_LO16);
+  }
+
   // We really must not split an IT block.
   LLVM_DEBUG(unsigned PredReg; assert(
                  !isThumb || getITInstrPredicate(*MI, PredReg) == ARMCC::AL));