[X86] Fix tls variable lowering issue with large code model

[LuoYuanke]

Summary

The problem here is the lowering for tls variable. Below is the DAG for the code.
SelectionDAG has 11 nodes:

t0: ch = EntryToken

    t8: i64,ch = load<(load 8 from `i8 addrspace(257)* null`, addrspace 257)> t0, Constant:i64<0>, undef:i64
      t10: i64 = X86ISD::WrapperRIP TargetGlobalTLSAddress:i64<i32* @x> 0 [TF=10]
    t11: i64,ch = load<(load 8 from got)> t0, t10, undef:i64
  t12: i64 = add t8, t11
t4: i32,ch = load<(dereferenceable load 4 from @x)> t0, t12, undef:i64

t6: ch = CopyToReg t0, Register:i32 %0, t4
And when mcmodel is large, below instruction can NOT be folded.

t10: i64 = X86ISD::WrapperRIP TargetGlobalTLSAddress:i64<i32* @x> 0 [TF=10]

t11: i64,ch = load<(load 8 from got)> t0, t10, undef:i64
So "t11: i64,ch = load<(load 8 from got)> t0, t10, undef:i64" is lowered to " Morphed node: t11: i64,ch = MOV64rm<Mem:(load 8 from got)> t10, TargetConstant:i8<1>, Register:i64 $noreg, TargetConstant:i32<0>, Register:i32 $noreg, t0"

When llvm start to lower "t10: i64 = X86ISD::WrapperRIP TargetGlobalTLSAddress:i64<i32* @x> 0 [TF=10]", it fails.

The patch is to fold the load and X86ISD::WrapperRIP.

Fixes PR26906

[LuoYuanke]

Hi rnk,
Could you help to review? Thanks

[rnk]

lib/Target/X86/X86ISelDAGToDAG.cpp

1150 ↗

(On Diff #187182)

"a mode"

test/CodeGen/X86/code-model-elf.ll

378 ↗

(On Diff #187182)

These all look the same. Is that correct for the large code model? I thought in the large code model the GOT would be in a PIC register, which I don't see below. Is that known? Maybe there should be a FIXME comment here if you don't consider that to be in scope of this change.

411 ↗

(On Diff #187182)

Wouldn't this load need to be through the PIC register?

[LuoYuanke]

test/CodeGen/X86/code-model-elf.ll

411 ↗

(On Diff #187182)

Seems no, because in the test case, it specify the pie option "!2 = !{i32 7, !"PIE Level", i32 2}". We can generate the same result with gcc.

bash-4.2$ cat t.c
extern __thread int x;
int f(void) { return x; }

bash-4.2$ gcc -S -fpic -fpie t.c -mcmodel=large -o -

.file   "t.c"
.text
.globl  f
.type   f, @function

f:
.LFB0:

.cfi_startproc
pushq   %rbp
.cfi_def_cfa_offset 16
.cfi_offset 6, -16
movq    %rsp, %rbp
.cfi_def_cfa_register 6
movq    x@gottpoff(%rip), %rax
movl    %fs:(%rax), %eax
popq    %rbp
.cfi_def_cfa 7, 8
ret
.cfi_endproc

.LFE0:

.size   f, .-f
.ident  "GCC: (GNU) 4.8.5 20150623 (Red Hat 4.8.5-16)"
.section        .note.GNU-stack,"",@progbits

[LuoYuanke]

Fix typo of comments.

[LuoYuanke]

lib/Target/X86/X86ISelDAGToDAG.cpp

1150 ↗

(On Diff #187182)

Fixed. Thanks.

[LuoYuanke]

test/CodeGen/X86/code-model-elf.ll

378 ↗

(On Diff #187182)

The result is same because we specify -fpie option in the test code. I notice tls large model for pic support is still not accurate, but it should be fixed in another patch. The patch focus on static tls or initial exec tls. The idea to add a FIXME comment is good. I'll do it.

[LuoYuanke]

Add "FIXME" for tls compiled with pic but without pie option.

[LuoYuanke]

@rnk
Do you agree to just fix the " X86ISD::WrapperRIP TargetGlobalTLSAddress" issue in this patch? Do you have some other concerns or comments for the patch?

[rnk]

lgtm

test/CodeGen/X86/code-model-elf.ll

411 ↗

(On Diff #187182)

Hm, perhaps we really ought to fix the "PIC" checks to say "PIE".

llvm/trunk/lib/Target/X86/X86ISelDAGToDAG.cpp

 /// mode. These wrap things that will resolve down into a symbol reference.
 /// If no match is possible, this returns true, otherwise it returns false.
 bool X86DAGToDAGISel::matchWrapper(SDValue N, X86ISelAddressMode &AM) {
   // If the addressing mode already has a symbol as the displacement, we can
   // never match another symbol.
   if (AM.hasSymbolicDisplacement())
     return true;
 
+  bool IsRIPRelTLS = false;
   bool IsRIPRel = N.getOpcode() == X86ISD::WrapperRIP;
+  if (IsRIPRel) {
+    SDValue Val = N.getOperand(0);
+    if (Val.getOpcode() == ISD::TargetGlobalTLSAddress)
+      IsRIPRelTLS = true;
+  }
 
-  // We can't use an addressing mode in the 64-bit large code model. In the
-  // medium code model, we use can use an mode when RIP wrappers are present.
-  // That signifies access to globals that are known to be "near", such as the
-  // GOT itself.
+  // We can't use an addressing mode in the 64-bit large code model.
+  // Global TLS addressing is an exception. In the medium code model,
+  // we use can use a mode when RIP wrappers are present.
+  // That signifies access to globals that are known to be "near",
+  // such as the GOT itself.
   CodeModel::Model M = TM.getCodeModel();
   if (Subtarget->is64Bit() &&
-      (M == CodeModel::Large || (M == CodeModel::Medium && !IsRIPRel)))
+      ((M == CodeModel::Large && !IsRIPRelTLS) ||
+       (M == CodeModel::Medium && !IsRIPRel)))
     return true;
 
   // Base and index reg must be 0 in order to use %rip as base.
   if (IsRIPRel && AM.hasBaseOrIndexReg())
     return true;
 
   // Make a local copy in case we can't do this fold.
   X86ISelAddressMode Backup = AM;

llvm/trunk/test/CodeGen/X86/code-model-elf.ll

 ; ModuleID = 'model.c'
 source_filename = "model.c"
 target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64--linux"
 
 @global_data = dso_local global [10 x i32] [i32 1, i32 2, i32 0, i32 0, i32 0, i32 0, i32 0, i32 0, i32 0, i32 0], align 16
 @static_data = internal global [10 x i32] zeroinitializer, align 16
 @extern_data = external global [10 x i32], align 16
+@thread_data = external thread_local global i32, align 4
+
 
 define dso_local i32* @lea_static_data() #0 {
 ; SMALL-STATIC-LABEL: lea_static_data:
 ; SMALL-STATIC:       # %bb.0:
 ; SMALL-STATIC-NEXT:    movl $static_data, %eax
 ; SMALL-STATIC-NEXT:    retq
 ;
 ; MEDIUM-STATIC-LABEL: lea_static_data:

 ; LARGE-PIC-NEXT:    movabsq $_GLOBAL_OFFSET_TABLE_-.L9$pb, %rcx
 ; LARGE-PIC-NEXT:    addq %rax, %rcx
 ; LARGE-PIC-NEXT:    movabsq $extern_fn@GOT, %rax
 ; LARGE-PIC-NEXT:    movq (%rcx,%rax), %rax
 ; LARGE-PIC-NEXT:    retq
   ret void ()* @extern_fn
 }
 
+; FIXME: The result is same for small, medium and large model, because we
+; specify pie option in the test case. And the type of tls is initial exec tls.
+; For pic code. The large model code for pic tls should be emitted as below.
+
+; .L3:
+; leaq  .L3(%rip), %rbx
+; movabsq $_GLOBAL_OFFSET_TABLE_-.L3, %r11
+; addq  %r11, %rbx
+; leaq  thread_data@TLSGD(%rip), %rdi
+; movabsq $__tls_get_addr@PLTOFF, %rax
+; addq  %rbx, %rax
+; call  *%rax
+; movl  (%rax), %eax
+
+; The medium and small model code for pic tls should be emitted as below.
+; data16
+; leaq  thread_data@TLSGD(%rip), %rdi
+; data16
+; data16
+; rex64
+; callq __tls_get_addr@PLT
+; movl  (%rax), %eax
+
+define dso_local i32 @load_thread_data() #0 {
+; SMALL-STATIC-LABEL: load_thread_data:
+; SMALL-STATIC:       # %bb.0:
+; SMALL-STATIC-NEXT:    movq    thread_data@GOTTPOFF(%rip), %rax
+; SMALL-STATIC-NEXT:    movl    %fs:(%rax), %eax
+; SMALL-STATIC-NEXT:    retq
+;
+; MEDIUM-STATIC-LABEL: load_thread_data:
+; MEDIUM-STATIC:       # %bb.0:
+; MEDIUM-STATIC-NEXT:    movq    thread_data@GOTTPOFF(%rip), %rax
+; MEDIUM-STATIC-NEXT:    movl    %fs:(%rax), %eax
+; MEDIUM-STATIC-NEXT:    retq
+;
+; LARGE-STATIC-LABEL: load_thread_data:
+; LARGE-STATIC:       # %bb.0:
+; LARGE-STATIC-NEXT:    movq    thread_data@GOTTPOFF(%rip), %rax
+; LARGE-STATIC-NEXT:    movl    %fs:(%rax), %eax
+; LARGE-STATIC-NEXT:    retq
+;
+; SMALL-PIC-LABEL: load_thread_data:
+; SMALL-PIC:       # %bb.0:
+; SMALL-PIC-NEXT:    movq    thread_data@GOTTPOFF(%rip), %rax
+; SMALL-PIC-NEXT:    movl    %fs:(%rax), %eax
+; SMALL-PIC-NEXT:    retq
+;
+; MEDIUM-PIC-LABEL: load_thread_data:
+; MEDIUM-PIC:       # %bb.0:
+; MEDIUM-PIC-NEXT:    movq    thread_data@GOTTPOFF(%rip), %rax
+; MEDIUM-PIC-NEXT:    movl    %fs:(%rax), %eax
+; MEDIUM-PIC-NEXT:    retq
+;
+; LARGE-PIC-LABEL: load_thread_data:
+; LARGE-PIC:       # %bb.0:
+; LARGE-PIC-NEXT:    movq    thread_data@GOTTPOFF(%rip), %rax
+; LARGE-PIC-NEXT:    movl    %fs:(%rax), %eax
+; LARGE-PIC-NEXT:    retq
+;
+  %1 = load i32, i32* @thread_data, align 4
+  ret i32 %1
+}
+
 attributes #0 = { noinline nounwind uwtable }
 
 !llvm.module.flags = !{!0, !1, !2}
 !llvm.ident = !{!3}
 
 !0 = !{i32 1, !"wchar_size", i32 4}
 !1 = !{i32 7, !"PIC Level", i32 2}
 !2 = !{i32 7, !"PIE Level", i32 2}
 !3 = !{!"clang version 7.0.0 "}