[AArch 64] Use a constant pool load for weak symbol references when using static relocation model and small code model

[rmaprath]

Summary

Original patch email: http://thread.gmane.org/gmane.comp.compilers.llvm.cvs/203866 (from Keith Walker)

Currently we generate GOT based relocations for weak symbol references regardless of the underlying relocation model. This should be change so that in static relocation model we use a constant pool load instead. This is a regression from ARM64 - AArch64 merge (it was a constant pool load before).

[rengolin]

As discussed in the list, with the minor comment, LGTM. Thanks!

lib/Target/AArch64/AArch64ISelLowering.cpp

2739

Would be good to add a comment here to state that the ranges are big enough so that this load doesn't end up with the same problem you were trying to fix in the first place.

[rmaprath]

Thanks both!

Committed in r217503.

lib/Target/AArch64/AArch64FastISel.cpp

       .addReg(ADRPReg)
       .addGlobalAddress(GV, 0, AArch64II::MO_GOT | AArch64II::MO_PAGEOFF |
                         AArch64II::MO_NC);
+  } else if (OpFlags & AArch64II::MO_CONSTPOOL) {
+    // We can't handle addresses loaded from a constant pool quickly yet.
+    return 0;
   } else {
     // ADRP + ADDX
     BuildMI(*FuncInfo.MBB, FuncInfo.InsertPt, DbgLoc, TII.get(AArch64::ADRP),

lib/Target/AArch64/AArch64ISelLowering.cpp

                                                   SelectionDAG &DAG) const {
   EVT PtrVT = getPointerTy();
   SDLoc DL(Op);
-  const GlobalValue *GV = cast<GlobalAddressSDNode>(Op)->getGlobal();
+  const GlobalAddressSDNode *GN = cast<GlobalAddressSDNode>(Op);
+  const GlobalValue *GV = GN->getGlobal();
   unsigned char OpFlags =
       Subtarget->ClassifyGlobalReference(GV, getTargetMachine());
 

     return DAG.getNode(AArch64ISD::LOADgot, DL, PtrVT, GotAddr);
   }
 
+  if ((OpFlags & AArch64II::MO_CONSTPOOL) != 0) {
+    assert(getTargetMachine().getCodeModel() == CodeModel::Small &&
+           "use of MO_CONSTPOOL only supported on small model");
+    SDValue Hi = DAG.getTargetConstantPool(GV, PtrVT, 0, 0, AArch64II::MO_PAGE);
+    unsigned char LoFlags = AArch64II::MO_PAGEOFF | AArch64II::MO_NC;
+    SDValue Lo = DAG.getTargetConstantPool(GV, PtrVT, 0, 0, LoFlags);
+
+    SDValue ADRP = DAG.getNode(AArch64ISD::ADRP, DL, PtrVT, Hi);
+    SDValue PoolAddr = DAG.getNode(AArch64ISD::ADDlow, DL, PtrVT, ADRP, Lo);
+    SDValue GlobalAddr = DAG.getLoad(PtrVT, DL, DAG.getEntryNode(), PoolAddr,
+                                     MachinePointerInfo::getConstantPool(),
+                                     /*isVolatile=*/ false,
+                                     /*isNonTemporal=*/ true,
+                                     /*isInvariant=*/ true, 8);
+    if (GN->getOffset() != 0)
+      return DAG.getNode(ISD::ADD, DL, PtrVT, GlobalAddr,
+                         DAG.getConstant(GN->getOffset(), PtrVT));
+    return GlobalAddr;
+  }
+
   if (getTargetMachine().getCodeModel() == CodeModel::Large) {
     const unsigned char MO_NC = AArch64II::MO_NC;
     return DAG.getNode(

lib/Target/AArch64/AArch64Subtarget.cpp

     return AArch64II::MO_GOT;
 
   // The small code mode's direct accesses use ADRP, which cannot necessarily
-  // produce the value 0 (if the code is above 4GB). Therefore they must use the
-  // GOT.
-  if (TM.getCodeModel() == CodeModel::Small && GV->isWeakForLinker() && isDecl)
-    return AArch64II::MO_GOT;
+  // produce the value 0 (if the code is above 4GB).
+  if (TM.getCodeModel() == CodeModel::Small &&
+      GV->isWeakForLinker() && isDecl) {
+    // In PIC mode use the GOT, but in absolute mode use a constant pool load.
+    if (TM.getRelocationModel() == Reloc::Static)
+        return AArch64II::MO_CONSTPOOL;
+    else
+        return AArch64II::MO_GOT;
+  }
 
   // If symbol visibility is hidden, the extra load is not needed if
   // the symbol is definitely defined in the current translation unit.

lib/Target/AArch64/Utils/AArch64BaseInfo.h

     /// thread-local symbol. On Darwin, only one type of thread-local access
     /// exists (pre linker-relaxation), but on ELF the TLSModel used for the
     /// referee will affect interpretation.
-    MO_TLS = 0x20
+    MO_TLS = 0x20,
+
+    /// MO_CONSTPOOL - This flag indicates that a symbol operand represents
+    /// the address of a constant pool entry for the symbol, rather than the
+    /// address of the symbol itself.
+    MO_CONSTPOOL = 0x40
   };
 } // end namespace AArch64II
 

test/CodeGen/AArch64/arm64-extern-weak.ll

-; RUN: llc -mtriple=arm64-none-linux-gnu -o - < %s | FileCheck %s
+; RUN: llc -mtriple=arm64-none-linux-gnu -relocation-model=pic -o - < %s | FileCheck %s
+; RUN: llc -mtriple=arm64-none-linux-gnu -relocation-model=static -o - < %s | FileCheck --check-prefix=CHECK-STATIC %s
 ; RUN: llc -mtriple=arm64-none-linux-gnu -code-model=large -o - < %s | FileCheck --check-prefix=CHECK-LARGE %s
 
 declare extern_weak i32 @var()
 
 define i32()* @foo() {
 ; The usual ADRP/ADD pair can't be used for a weak reference because it must
-; evaluate to 0 if the symbol is undefined. We use a litpool entry.
+; evaluate to 0 if the symbol is undefined. We use a GOT entry for PIC
+; otherwise a litpool entry.
   ret i32()* @var
 
 ; CHECK: adrp x[[VAR:[0-9]+]], :got:var
 ; CHECK: ldr x0, [x[[VAR]], :got_lo12:var]
 
+; CHECK-STATIC: .LCPI0_0:
+; CHECK-STATIC-NEXT: .xword  var
+; CHECK-STATIC: adrp x[[VAR:[0-9]+]], .LCPI0_0
+; CHECK-STATIC: ldr x0, [x[[VAR]], :lo12:.LCPI0_0]
+
   ; In the large model, the usual relocations are absolute and can
   ; materialise 0.
 ; CHECK-LARGE: movz x0, #:abs_g3:var

 ; CHECK: add x0, [[ARR_VAR]], #20
   ret i32* %addr
 
+; CHECK-STATIC: .LCPI1_0:
+; CHECK-STATIC-NEXT: .xword arr_var
+; CHECK-STATIC: ldr [[BASE:x[0-9]+]], [{{x[0-9]+}}, :lo12:.LCPI1_0]
+; CHECK-STATIC: add x0, [[BASE]], #20
+
   ; In the large model, the usual relocations are absolute and can
   ; materialise 0.
 ; CHECK-LARGE: movz [[ARR_VAR:x[0-9]+]], #:abs_g3:arr_var

 ; CHECK: adrp [[BASE:x[0-9]+]], defined_weak_var
 ; CHECK: add x0, [[BASE]], :lo12:defined_weak_var
 
+; CHECK-STATIC: adrp [[BASE:x[0-9]+]], defined_weak_var
+; CHECK-STATIC: add x0, [[BASE]], :lo12:defined_weak_var
+
 ; CHECK-LARGE: movz x0, #:abs_g3:defined_weak_var
 ; CHECK-LARGE: movk x0, #:abs_g2_nc:defined_weak_var
 ; CHECK-LARGE: movk x0, #:abs_g1_nc:defined_weak_var

test/CodeGen/AArch64/extern-weak.ll

-; RUN: llc -mtriple=aarch64-none-linux-gnu -o - %s | FileCheck %s
+; RUN: llc -mtriple=aarch64-none-linux-gnu -relocation-model=pic -o - %s | FileCheck %s
+; RUN: llc -mtriple=aarch64-none-linux-gnu -relocation-model=static -o - < %s | FileCheck --check-prefix=CHECK-STATIC %s
 ; RUN: llc -mtriple=aarch64-none-linux-gnu -code-model=large -o - %s | FileCheck --check-prefix=CHECK-LARGE %s
 
 declare extern_weak i32 @var()
 
 define i32()* @foo() {
 ; The usual ADRP/ADD pair can't be used for a weak reference because it must
-; evaluate to 0 if the symbol is undefined. We use a litpool entry.
+; evaluate to 0 if the symbol is undefined. We use a GOT entry for PIC
+; otherwise a litpool entry.
   ret i32()* @var
 
 
 ; CHECK: adrp x[[ADDRHI:[0-9]+]], :got:var
 ; CHECK: ldr x0, [x[[ADDRHI]], :got_lo12:var]
 
+; CHECK-STATIC: .LCPI0_0:
+; CHECK-STATIC-NEXT: .xword  var
+; CHECK-STATIC: adrp x[[VAR:[0-9]+]], .LCPI0_0
+; CHECK-STATIC: ldr x0, [x[[VAR]], :lo12:.LCPI0_0]
+
   ; In the large model, the usual relocations are absolute and can
   ; materialise 0.
 ; CHECK-LARGE: movz x0, #:abs_g3:var

 ; CHECK: ldr [[BASE:x[0-9]+]], [x[[ADDRHI]], :got_lo12:arr_var]
 ; CHECK: add x0, [[BASE]], #20
 
+; CHECK-STATIC: .LCPI1_0:
+; CHECK-STATIC-NEXT: .xword arr_var
+; CHECK-STATIC: ldr [[BASE:x[0-9]+]], [{{x[0-9]+}}, :lo12:.LCPI1_0]
+; CHECK-STATIC: add x0, [[BASE]], #20
+
   ret i32* %addr
 
   ; In the large model, the usual relocations are absolute and can

 ; CHECK: adrp [[BASE:x[0-9]+]], defined_weak_var
 ; CHECK: add x0, [[BASE]], :lo12:defined_weak_var
 
+; CHECK-STATIC: adrp [[BASE:x[0-9]+]], defined_weak_var
+; CHECK-STATIC: add x0, [[BASE]], :lo12:defined_weak_var
+
 ; CHECK-LARGE: movz x0, #:abs_g3:defined_weak_var
 ; CHECK-LARGE: movk x0, #:abs_g2_nc:defined_weak_var
 ; CHECK-LARGE: movk x0, #:abs_g1_nc:defined_weak_var