[PATCH] D64715: [WIP][RISCV] Use RISCV_32_PCREL reloc for FDE initial location

[Edward Jones via Phabricator via llvm-commits]

edward-jones created this revision.
edward-jones added a reviewer: asb.
Herald added subscribers: llvm-commits, Jim, benna, psnobl, jocewei, PkmX, rkruppe, the_o, brucehoult, MartinMosbeck, rogfer01, zzheng, MaskRay, jrtc27, shiva0217, kito-cheng, niosHD, sabuasal, apazos, simoncook, johnrusso, rbar.
Herald added a project: LLVM.

The default FDE encoding is a 4-byte pc-relative offset, and the standard implementation of MCAsmInfo::getExprForFDESymbol generates a <symbol>-<pc> expression for this (which generates a R_RISCV_ADD32+R_RISCV_SUB32 reloc pair).

      

For reasons that are still not quite clear, the GNU toolchain generates a R_RISCV_32_PCREL relocation instead of the R_RISCV_ADD32+R_RISCV_SUB32 pair. This patch matches that behavior.


Repository:
  rL LLVM

https://reviews.llvm.org/D64715

Files:
  include/llvm/MC/MCExpr.h
  lib/MC/MCExpr.cpp
  lib/Target/RISCV/MCTargetDesc/RISCVELFObjectWriter.cpp
  lib/Target/RISCV/MCTargetDesc/RISCVMCAsmInfo.cpp
  lib/Target/RISCV/MCTargetDesc/RISCVMCAsmInfo.h


Index: lib/Target/RISCV/MCTargetDesc/RISCVMCAsmInfo.h
===================================================================
--- lib/Target/RISCV/MCTargetDesc/RISCVMCAsmInfo.h
+++ lib/Target/RISCV/MCTargetDesc/RISCVMCAsmInfo.h
@@ -23,6 +23,10 @@
 
 public:
   explicit RISCVMCAsmInfo(const Triple &TargetTriple);
+
+  const MCExpr* getExprForFDESymbol(const MCSymbol *Sym,
+                                    unsigned Encoding,
+                                    MCStreamer &Streamer) const override;
 };
 
 } // namespace llvm
Index: lib/Target/RISCV/MCTargetDesc/RISCVMCAsmInfo.cpp
===================================================================
--- lib/Target/RISCV/MCTargetDesc/RISCVMCAsmInfo.cpp
+++ lib/Target/RISCV/MCTargetDesc/RISCVMCAsmInfo.cpp
@@ -12,6 +12,8 @@
 
 #include "RISCVMCAsmInfo.h"
 #include "llvm/ADT/Triple.h"
+#include "llvm/BinaryFormat/Dwarf.h"
+#include "llvm/MC/MCStreamer.h"
 using namespace llvm;
 
 void RISCVMCAsmInfo::anchor() {}
@@ -25,3 +27,14 @@
   Data16bitsDirective = "\t.half\t";
   Data32bitsDirective = "\t.word\t";
 }
+
+const MCExpr*
+RISCVMCAsmInfo::getExprForFDESymbol(const MCSymbol *Sym,
+                                    unsigned Encoding,
+                                    MCStreamer &Streamer) const {
+  auto &Ctx = Streamer.getContext();
+  if (Encoding | dwarf::DW_EH_PE_pcrel)
+    return MCSymbolRefExpr::create(Sym, MCSymbolRefExpr::VK_RISCV_PCREL, Ctx);
+  else
+    return MCSymbolRefExpr::create(Sym, Ctx);
+}
Index: lib/Target/RISCV/MCTargetDesc/RISCVELFObjectWriter.cpp
===================================================================
--- lib/Target/RISCV/MCTargetDesc/RISCVELFObjectWriter.cpp
+++ lib/Target/RISCV/MCTargetDesc/RISCVELFObjectWriter.cpp
@@ -10,6 +10,7 @@
 #include "MCTargetDesc/RISCVMCTargetDesc.h"
 #include "llvm/MC/MCELFObjectWriter.h"
 #include "llvm/MC/MCFixup.h"
+#include "llvm/MC/MCValue.h"
 #include "llvm/MC/MCObjectWriter.h"
 #include "llvm/Support/ErrorHandling.h"
 
@@ -48,11 +49,15 @@
                                             const MCFixup &Fixup,
                                             bool IsPCRel) const {
   // Determine the type of the relocation
+  MCSymbolRefExpr::VariantKind Variant = Target.getAccessVariant();
   switch ((unsigned)Fixup.getKind()) {
   default:
     llvm_unreachable("invalid fixup kind!");
   case FK_Data_4:
-    return ELF::R_RISCV_32;
+    if (Variant == MCSymbolRefExpr::VK_RISCV_PCREL)
+      return ELF::R_RISCV_32_PCREL;
+    else
+      return ELF::R_RISCV_32;
   case FK_Data_8:
     return ELF::R_RISCV_64;
   case FK_Data_Add_1:
Index: lib/MC/MCExpr.cpp
===================================================================
--- lib/MC/MCExpr.cpp
+++ lib/MC/MCExpr.cpp
@@ -312,6 +312,7 @@
   case VK_AMDGPU_REL64: return "rel64";
   case VK_AMDGPU_ABS32_LO: return "abs32 at lo";
   case VK_AMDGPU_ABS32_HI: return "abs32 at hi";
+  case VK_RISCV_PCREL: return "pcrel";
   }
   llvm_unreachable("Invalid variant kind");
 }
Index: include/llvm/MC/MCExpr.h
===================================================================
--- include/llvm/MC/MCExpr.h
+++ include/llvm/MC/MCExpr.h
@@ -296,6 +296,8 @@
     VK_AMDGPU_ABS32_LO,      // symbol at abs32@lo
     VK_AMDGPU_ABS32_HI,      // symbol at abs32@hi
 
+    VK_RISCV_PCREL,
+
     VK_TPREL,
     VK_DTPREL
   };


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