[llvm] Support indirect symbol replacement with GOTPCREL for x86_64 ELF

[PiJoules]

There's an existing check in LLVM that can replace an offset to a DSO-local symbol that is just a pointer to another symbol with a GOTPCREL reloc. This is exactly what the RTTI proxy in the relative vtables ABI is. This feature is supported for different macho platforms, but not for ELF. This extends that support.

Additionally allow negative offsets before checking if we can support GOTPCREL. These offsets can be negative.

[llvmbot]

@llvm/pr-subscribers-mc

@llvm/pr-subscribers-backend-x86

Changes

There's an existing check in LLVM that can replace an offset to a DSO-local symbol that is just a pointer to another symbol with a GOTPCREL reloc. This is exactly what the RTTI proxy in the relative vtables ABI is. This feature is supported for different macho platforms, but not for ELF. This extends that support.

Additionally allow negative offsets before checking if we can support GOTPCREL. These offsets can be negative.

---

Full diff: https://github.com/llvm/llvm-project/pull/67754.diff

4 Files Affected:

• (modified) llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp (-5)
• (modified) llvm/lib/Target/X86/X86TargetObjectFile.cpp (+10)
• (modified) llvm/lib/Target/X86/X86TargetObjectFile.h (+7)
• (added) llvm/test/MC/ELF/rtti-proxy-gotpcrel.ll (+13)

diff --git a/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp b/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
index 0c4ea1b3d9f04d9..4a79c6157b17125 100644
--- a/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
@@ -3479,12 +3479,7 @@ static void handleIndirectSymViaGOTPCRel(AsmPrinter &AP, const MCExpr **ME,
   //
   //    gotpcrelcst := <offset from @foo base> + <cst>
   //
-  // If gotpcrelcst is positive it means that we can safely fold the pc rel
-  // displacement into the GOTPCREL. We can also can have an extra offset <cst>
-  // if the target knows how to encode it.
   int64_t GOTPCRelCst = Offset + MV.getConstant();
-  if (GOTPCRelCst < 0)
-    return;
   if (!AP.getObjFileLowering().supportGOTPCRelWithOffset() && GOTPCRelCst != 0)
     return;
 
diff --git a/llvm/lib/Target/X86/X86TargetObjectFile.cpp b/llvm/lib/Target/X86/X86TargetObjectFile.cpp
index b88ad5a478f390a..53c692060f08cf1 100644
--- a/llvm/lib/Target/X86/X86TargetObjectFile.cpp
+++ b/llvm/lib/Target/X86/X86TargetObjectFile.cpp
@@ -56,3 +56,13 @@ const MCExpr *X86ELFTargetObjectFile::getDebugThreadLocalSymbol(
     const MCSymbol *Sym) const {
   return MCSymbolRefExpr::create(Sym, MCSymbolRefExpr::VK_DTPOFF, getContext());
 }
+
+const MCExpr *X86ELFTargetObjectFile::getIndirectSymViaGOTPCRel(
+    const GlobalValue *GV, const MCSymbol *Sym, const MCValue &MV,
+    int64_t Offset, MachineModuleInfo *MMI, MCStreamer &Streamer) const {
+  int64_t FinalOffset = Offset + MV.getConstant();
+  const MCExpr *Res =
+      MCSymbolRefExpr::create(Sym, MCSymbolRefExpr::VK_GOTPCREL, getContext());
+  const MCExpr *Off = MCConstantExpr::create(FinalOffset, getContext());
+  return MCBinaryExpr::createAdd(Res, Off, getContext());
+}
diff --git a/llvm/lib/Target/X86/X86TargetObjectFile.h b/llvm/lib/Target/X86/X86TargetObjectFile.h
index f4bf52c83771ff4..ed9390d1fad1a26 100644
--- a/llvm/lib/Target/X86/X86TargetObjectFile.h
+++ b/llvm/lib/Target/X86/X86TargetObjectFile.h
@@ -42,9 +42,16 @@ namespace llvm {
   public:
     X86ELFTargetObjectFile() {
       PLTRelativeVariantKind = MCSymbolRefExpr::VK_PLT;
+      SupportIndirectSymViaGOTPCRel = true;
     }
     /// Describe a TLS variable address within debug info.
     const MCExpr *getDebugThreadLocalSymbol(const MCSymbol *Sym) const override;
+
+    const MCExpr *
+    getIndirectSymViaGOTPCRel(const GlobalValue *GV, const MCSymbol *Sym,
+                              const MCValue &MV, int64_t Offset,
+                              MachineModuleInfo *MMI,
+                              MCStreamer &Streamer) const override;
   };
 
 } // end namespace llvm
diff --git a/llvm/test/MC/ELF/rtti-proxy-gotpcrel.ll b/llvm/test/MC/ELF/rtti-proxy-gotpcrel.ll
new file mode 100644
index 000000000000000..a61af96b399895f
--- /dev/null
+++ b/llvm/test/MC/ELF/rtti-proxy-gotpcrel.ll
@@ -0,0 +1,13 @@
+; RUN: llc %s -mtriple=x86_64-unknown-fuchsia  -o - | FileCheck %s
+
+@vtable = dso_local unnamed_addr constant i32 trunc (i64 sub (i64 ptrtoint (i8** @rtti.proxy to i64), i64 ptrtoint (i32* @vtable to i64)) to i32), align 4
+@vtable_with_offset = dso_local unnamed_addr constant { [2 x i32] } { [2 x i32] [i32 0, i32 trunc (i64 sub (i64 ptrtoint (i8** @rtti.proxy to i64), i64 ptrtoint ({ [2 x i32] }* @vtable_with_offset to i64)) to i32)] }, align 4
+@rtti = external global i8, align 8
+@rtti.proxy = linkonce_odr hidden unnamed_addr constant i8* @rtti
+
+; CHECK-NOT: rtti.proxy
+; CHECK-LABEL: vtable:
+; CHECK:         .{{word|long}}   rtti@GOTPCREL+0{{$}}
+
+; CHECK-LABEL: vtable_with_offset:
+; CHECK:         .{{word|long}}   rtti@GOTPCREL+4{{$}}

[petrhosek]

Can you split the change to allow negative offsets to a separate PR and add a test for it?

[PiJoules]

Can you split the change to allow negative offsets to a separate PR and add a test for it?

#72308