[PATCH] D36280: Fix access to undefined weak symbols in pic code

[Rafael Ávila de Espíndola via Phabricator via llvm-commits]

rafael created this revision.

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.


https://reviews.llvm.org/D36280

Files:
  lib/Target/X86/X86Subtarget.cpp
  test/CodeGen/X86/weak-undef.ll


Index: test/CodeGen/X86/weak-undef.ll
===================================================================
--- /dev/null
+++ test/CodeGen/X86/weak-undef.ll
@@ -0,0 +1,45 @@
+; RUN: llc < %s -relocation-model=pic -mtriple=x86_64-pc-linux | FileCheck %s
+
+ at foo1 = extern_weak hidden global i32, align 4
+define i32* @bar1() {
+  ret i32* @foo1
+}
+; CHECK: bar1:
+; CHECK: movq foo1 at GOTPCREL(%rip), %rax
+
+ at 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 at GOTPCREL(%rip), %rax
+
+declare external hidden i32 @foo6()
+define i32()* @bar6() {
+  ret i32()* @foo6
+}
+; CHECK: bar6:
+; CHECK: leaq foo6(%rip), %rax
Index: lib/Target/X86/X86Subtarget.cpp
===================================================================
--- lib/Target/X86/X86Subtarget.cpp
+++ lib/Target/X86/X86Subtarget.cpp
@@ -103,6 +103,22 @@
   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.
@@ -122,7 +138,7 @@
     }
   }
 
-  if (TM.shouldAssumeDSOLocal(M, GV))
+  if (shouldAssumeGlobalReferenceLocal(this, TM, M, GV))
     return classifyLocalReference(GV);
 
   if (isTargetCOFF())


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