[PATCH] D36639: More fixes for weak undef and pic

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

rafael created this revision.

I am going on vacation in a few hours and will not have time to finish this. Feel free to take over.

It turns out that at least on relocations that have produce a 0 at runtime, we cannot assume it is local. If we do, the architecture dependent part will assume it can always compute 0 from a position in the file. While 0 might always be in range, it will have a different offset in different executions.

If this goes in as is, there will be some code that is redundant and can be cleaned up.


https://reviews.llvm.org/D36639

Files:
  lib/Target/TargetMachine.cpp
  test/CodeGen/X86/global-access-pie-copyrelocs.ll
  test/CodeGen/X86/hidden-vis-3.ll
  test/CodeGen/X86/weak-undef.ll


Index: test/CodeGen/X86/weak-undef.ll
===================================================================
--- test/CodeGen/X86/weak-undef.ll
+++ test/CodeGen/X86/weak-undef.ll
@@ -8,7 +8,7 @@
 ; CHECK: bar1:
 ; CHECK: movq foo1 at GOTPCREL(%rip), %rax
 ; I386: bar1:
-; I386: leal foo1 at GOTOFF(%eax), %eax
+; I386: movl foo1 at GOT(%eax), %eax
 
 @foo2 = external hidden global i32, align 4
 define i32* @bar2() {
@@ -25,9 +25,9 @@
   ret void
 }
 ; CHECK: bar3:
-; CHECK: callq foo3
+; CHECK: callq foo3 at PLT
 ; I386: bar3:
-; I386: calll foo3
+; I386: calll foo3 at PLT
 
 declare external hidden void @foo4()
 define void @bar4() {
@@ -46,7 +46,7 @@
 ; CHECK: bar5:
 ; CHECK: movq foo5 at GOTPCREL(%rip), %rax
 ; I386: bar5:
-; I386: leal foo5 at GOTOFF(%eax), %eax
+; I386: movl foo5 at GOT(%eax), %eax
 
 declare external hidden i32 @foo6()
 define i32()* @bar6() {
Index: test/CodeGen/X86/hidden-vis-3.ll
===================================================================
--- test/CodeGen/X86/hidden-vis-3.ll
+++ test/CodeGen/X86/hidden-vis-3.ll
@@ -10,7 +10,8 @@
 ; X32: movl _y, %eax
 
 ; X64: _t:
-; X64: movl _y(%rip), %eax
+; X64: movq _y at GOTPCREL(%rip), %rax
+; X64: movl (%rax), %eax
 
 	%0 = load i32, i32* @x, align 4		; <i32> [#uses=1]
 	%1 = load i32, i32* @y, align 4		; <i32> [#uses=1]
Index: test/CodeGen/X86/global-access-pie-copyrelocs.ll
===================================================================
--- test/CodeGen/X86/global-access-pie-copyrelocs.ll
+++ test/CodeGen/X86/global-access-pie-copyrelocs.ll
@@ -63,6 +63,20 @@
   ret i32 %0
 }
 
+; ExternalWeak Linkage
+ at e = extern_weak global i32, align 4
+
+define i32* @my_access_global_d() #0 {
+; X32-LABEL: my_access_global_d:
+; X32:       addl $_GLOBAL_OFFSET_TABLE_{{.*}}, %eax
+; X32:       movl e at GOT(%eax), %eax
+; X64-LABEL: my_access_global_d:
+; X64:       movq e at GOTPCREL(%rip), %rax
+
+entry:
+  ret i32* @e
+}
+
 ; External Linkage, only declaration, store a value.
 
 define i32 @my_access_global_store_d() #0 {
Index: lib/Target/TargetMachine.cpp
===================================================================
--- lib/Target/TargetMachine.cpp
+++ lib/Target/TargetMachine.cpp
@@ -134,6 +134,16 @@
   if (TT.isOSBinFormatCOFF() || (TT.isOSWindows() && TT.isOSBinFormatMachO()))
     return true;
 
+  // A weak undef symbol might resolve to 0. We have to assume it is
+  // not local, as we have no control of where in memory we will end
+  // up.
+  // FIXME: is this true for all architectures?
+  // FIXME2: this is false for some relocations. For example, it should be valid
+  // to produce "call foo" instead of "call foo at plt". It is not clear if that is
+  // a problem.
+  if (GV && GV->hasExternalWeakLinkage() && isPositionIndependent())
+    return false;
+
   if (GV && (GV->hasLocalLinkage() || !GV->hasDefaultVisibility()))
     return true;
 


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