[ARM] Make RWPI use movw/movt when available

When constructing global address literals while targeting the RWPI
relocation model. LLVM currently only uses literal pools. If MOVW/MOVT
instructions are available we can use these instead. Beside being more
efficient it allows -arm-execute-only to work with
-relocation-model=RWPI as well.

When we generate MOVW/MOVT for global addresses when targeting the RWPI
relocation model, we need to use base relative relocations. This patch
does the needed plumbing in MC to generate these for MOVW/MOVT.

Differential Revision: https://reviews.llvm.org/D29487

Change-Id: I446786e43a6f5aa9b6a5bb2cd216d60d41c7755d
llvm-svn: 294298

1 files changed, 18 insertions, 2 deletions

diff --git a/llvm/lib/Target/ARM/MCTargetDesc/ARMELFObjectWriter.cpp b/llvm/lib/Target/ARM/MCTargetDesc/ARMELFObjectWriter.cpp
index 2c947541322..c921e4cf782 100644
--- a/llvm/lib/Target/ARM/MCTargetDesc/ARMELFObjectWriter.cpp
+++ b/llvm/lib/Target/ARM/MCTargetDesc/ARMELFObjectWriter.cpp
@@ -269,10 +269,26 @@ unsigned ARMELFObjectWriter::GetRelocTypeInner(const MCValue &Target,
       }
       break;
     case ARM::fixup_t2_movt_hi16:
-      Type = ELF::R_ARM_THM_MOVT_ABS;
+      switch (Modifier) {
+      default: llvm_unreachable("Unsupported Modifier");
+      case MCSymbolRefExpr::VK_None:
+        Type = ELF::R_ARM_THM_MOVT_ABS;
+        break;
+      case MCSymbolRefExpr::VK_ARM_SBREL:
+        Type = ELF:: R_ARM_THM_MOVT_BREL;
+        break;
+      }
       break;
     case ARM::fixup_t2_movw_lo16:
-      Type = ELF::R_ARM_THM_MOVW_ABS_NC;
+      switch (Modifier) {
+      default: llvm_unreachable("Unsupported Modifier");
+      case MCSymbolRefExpr::VK_None:
+        Type = ELF::R_ARM_THM_MOVW_ABS_NC;
+        break;
+      case MCSymbolRefExpr::VK_ARM_SBREL:
+        Type = ELF:: R_ARM_THM_MOVW_BREL_NC;
+        break;
+      }
       break;
     }
   }