Fix access to undefined weak symbols in pic code

[• rafael]

Summary

When the access to a weak symbol is not a call, the access has to be able to produce the value 0 at runtime.

We were sometimes producing code sequences where that was not possible if the code was leaded more than 4g away from 0.

[rsmith]

lib/Target/X86/X86Subtarget.cpp

110–111

It seems to me that the bug is in shouldAssumeDSOLocal: we should not assume we can use a copy relocation for a variable that might not be defined.

[rsmith]

lib/Target/X86/X86Subtarget.cpp

110–111

... that said, I think perhaps we would be better off with /both/ changes, since I think your patch also fixes cases where we were not going to use a copy relocation (the __attribute__((visibility("hidden"), weak)) case).

[rsmith]

This looks reasonable to me. Please consider whether we should take D36604 too.

lib/Target/X86/X86Subtarget.cpp

115–117

Should this also check is64Bit()?

[• rafael]

r310756

lib/Target/X86/X86Subtarget.cpp

if (GV && (GV->isDeclarationForLinker() || GV->hasCommonLinkage()))
       return X86II::MO_DARWIN_NONLAZY_PIC_BASE;
 
     return X86II::MO_PIC_BASE_OFFSET;
   }
 
   return X86II::MO_GOTOFF;
 }
 
+static bool shouldAssumeGlobalReferenceLocal(const X86Subtarget *ST,
+                                             const TargetMachine &TM,
+                                             const Module &M,
+                                             const GlobalValue *GV) {
+  if (!TM.shouldAssumeDSOLocal(M, GV))
+    return false;
+  // A weak reference can end up being 0. If the code can be more that 4g away
+  // from zero and we are using the small code model we have to treat it as non
+  // local.
+  if (GV && GV->hasExternalWeakLinkage() &&
+      TM.getCodeModel() == CodeModel::Small && TM.isPositionIndependent() &&
+      ST->isTargetELF())
+    return false;
+  return true;
+}
+
 unsigned char X86Subtarget::classifyGlobalReference(const GlobalValue *GV,
                                                     const Module &M) const {
   // Large model never uses stubs.
   if (TM.getCodeModel() == CodeModel::Large)
     return X86II::MO_NO_FLAG;
 
   // Absolute symbols can be referenced directly.
   if (GV) {
     if (Optional<ConstantRange> CR = GV->getAbsoluteSymbolRange()) {
       // See if we can use the 8-bit immediate form. Note that some instructions
       // will sign extend the immediate operand, so to be conservative we only
       // accept the range [0,128).
       if (CR->getUnsignedMax().ult(128))
         return X86II::MO_ABS8;
       else
         return X86II::MO_NO_FLAG;
     }
   }
 
-  if (TM.shouldAssumeDSOLocal(M, GV))
+  if (shouldAssumeGlobalReferenceLocal(this, TM, M, GV))
     return classifyLocalReference(GV);
 
   if (isTargetCOFF())
     return X86II::MO_DLLIMPORT;
 
   if (is64Bit())
     return X86II::MO_GOTPCREL;
 

test/CodeGen/X86/weak-undef.ll

+; RUN: llc < %s -relocation-model=pic -mtriple=x86_64-pc-linux | FileCheck %s
+
+@foo1 = extern_weak hidden global i32, align 4
+define i32* @bar1() {
+  ret i32* @foo1
+}
+; CHECK: bar1:
+; CHECK: movq foo1@GOTPCREL(%rip), %rax
+
+@foo2 = external hidden global i32, align 4
+define i32* @bar2() {
+  ret i32* @foo2
+}
+; CHECK: bar2:
+; CHECK: leaq foo2(%rip), %rax
+
+declare extern_weak hidden void @foo3()
+define void @bar3() {
+  call void @foo3()
+  ret void
+}
+; CHECK: bar3:
+; CHECK: callq foo3
+
+declare external hidden void @foo4()
+define void @bar4() {
+  call void @foo4()
+  ret void
+}
+; CHECK: bar4:
+; CHECK: callq foo4
+
+declare extern_weak hidden i32 @foo5()
+define i32()* @bar5() {
+  ret i32()* @foo5
+}
+; CHECK: bar5:
+; CHECK: movq foo5@GOTPCREL(%rip), %rax
+
+declare external hidden i32 @foo6()
+define i32()* @bar6() {
+  ret i32()* @foo6
+}
+; CHECK: bar6:
+; CHECK: leaq foo6(%rip), %rax