[PATCH] D45460: With -fno-plt, GOTPCREL references must use RIP

[Sriraman Tallam via Phabricator via llvm-commits]

tmsriram created this revision.
tmsriram added reviewers: rnk, espindola.

With -fno-plt, global value references can use GOTPCREL and RIP must be used.

A simple example that breaks with -fno-plt:

extern "C" 
{
__attribute__((weak)) int weakfunc();

int fp_weakfunc() {

  if (&weakfunc)
    return 1;
  return 0;

}
}

$ clang -O2 -fno-plt -S file.cc
$ cat file.s
...
fp_weakfunc:                            # @fp_weakfunc
	.cfi_startproc

%bb.0:
======

xorl	%eax, %eax
	cmpq	$0, weakfunc at GOTPCREL
...

That's a GOTPCREL reference without %rip.  This will force the linker to only patch it with the offset in the GOT and will get the exact opposite program behavior at run-time.  This patch fixes it.


https://reviews.llvm.org/D45460

Files:
  lib/Target/X86/X86ISelLowering.cpp
  lib/Target/X86/X86ISelLowering.h
  test/CodeGen/X86/no-plt.ll


Index: test/CodeGen/X86/no-plt.ll
===================================================================
--- test/CodeGen/X86/no-plt.ll
+++ test/CodeGen/X86/no-plt.ll
@@ -3,6 +3,13 @@
 ; RUN: llc < %s -mcpu=generic -mtriple=x86_64-linux-gnu \
 ; RUN:   | FileCheck -check-prefix=X64 --check-prefix=STATIC %s
 
+define dso_local i32 @fp_weakfunc() local_unnamed_addr #0 {
+entry:
+; X64: weakfunc at GOTPCREL(%rip)
+  ret i32 select (i1 icmp ne (i32 ()* @weakfunc, i32 ()* null), i32 1, i32 0)
+}
+declare extern_weak i32 @weakfunc() nonlazybind
+
 define void @memset_call(i8* nocapture %a, i8 %c, i32 %n) {
 ; X64: callq *memset at GOTPCREL(%rip)
   call void @llvm.memset.p0i8.i32(i8* %a, i8 %c, i32 %n, i1 false)
Index: lib/Target/X86/X86ISelLowering.h
===================================================================
--- lib/Target/X86/X86ISelLowering.h
+++ lib/Target/X86/X86ISelLowering.h
@@ -1201,7 +1201,8 @@
     SDValue LowerEXTRACT_VECTOR_ELT(SDValue Op, SelectionDAG &DAG) const;
     SDValue LowerINSERT_VECTOR_ELT(SDValue Op, SelectionDAG &DAG) const;
 
-    unsigned getGlobalWrapperKind(const GlobalValue *GV = nullptr) const;
+    unsigned getGlobalWrapperKind(const GlobalValue *GV = nullptr,
+                                  const unsigned char OpFlags = 0) const;
     SDValue LowerConstantPool(SDValue Op, SelectionDAG &DAG) const;
     SDValue LowerBlockAddress(SDValue Op, SelectionDAG &DAG) const;
     SDValue LowerGlobalAddress(const GlobalValue *GV, const SDLoc &dl,
Index: lib/Target/X86/X86ISelLowering.cpp
===================================================================
--- lib/Target/X86/X86ISelLowering.cpp
+++ lib/Target/X86/X86ISelLowering.cpp
@@ -15497,7 +15497,8 @@
 }
 
 // Returns the appropriate wrapper opcode for a global reference.
-unsigned X86TargetLowering::getGlobalWrapperKind(const GlobalValue *GV) const {
+unsigned X86TargetLowering::getGlobalWrapperKind(
+    const GlobalValue *GV, const unsigned char OpFlags) const {
   // References to absolute symbols are never PC-relative.
   if (GV && GV->isAbsoluteSymbolRef())
     return X86ISD::Wrapper;
@@ -15507,6 +15508,16 @@
       (M == CodeModel::Small || M == CodeModel::Kernel))
     return X86ISD::WrapperRIP;
 
+  // GOTPCREL references are possible without PIC when using "nonlazybind"
+  // attribute.  Always use RIP when this is found.
+  if (OpFlags == X86II::MO_GOTPCREL) {
+    const Function *F = dyn_cast_or_null<Function>(GV);
+    assert(((F && F->hasFnAttribute(Attribute::NonLazyBind)) ||
+           (!F && GV->getParent()->getRtLibUseGOT())) &&
+           Subtarget.is64Bit());
+    return X86ISD::WrapperRIP;
+  }
+
   return X86ISD::Wrapper;
 }
 
@@ -15630,7 +15641,7 @@
     Result = DAG.getTargetGlobalAddress(GV, dl, PtrVT, 0, OpFlags);
   }
 
-  Result = DAG.getNode(getGlobalWrapperKind(GV), dl, PtrVT, Result);
+  Result = DAG.getNode(getGlobalWrapperKind(GV, OpFlags), dl, PtrVT, Result);
 
   // With PIC, the address is actually $g + Offset.
   if (isGlobalRelativeToPICBase(OpFlags)) {


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