[lld] [ELF/AArch64] Add GD to IE TLS relax optimization

[zatrazz]

Summary

This patch add the TLS Global-Dynamic to Initial-Exec TLS optimization
for AArch64. Similar of GD to LE, the idea is to optimize TLS access
to global symbols that can preempted by emitting a GOT entry and
a dynamic relocation (R_AARCH64_TLS_TPREL64).

With this patch, the R_AARCH64_TLSDESC* implementation [1]
(still in review), and a small one for aarch64 to fix the relativeRel
definition (which I plan to send for review shortly) I can bootstrap
and run all the lld tests without failure.

[grimar]

IMO that is cool if this helps to "bootstrap and run all the lld tests without failure" !

ELF/Target.cpp

1590

I think you missing ]

ldr     x1, [x0, #:tlsdesc_lo12:v ]

and "R_":

R_AARCH64_TLSDESC_ADD_LO12_NC

1591

.tlsdesccall    v

1598

Please remove the empty line.

1602

ditto.

1614

could you put return inside {...} to be consistent with other lld code ?

case R_AARCH64_TLSDESC_ADR_PAGE21: {
   // adrp
   uint64_t X = getAArch64Page(SA) - getAArch64Page(P);
   updateAArch64Addr(Loc, (X >> 12) & 0x1FFFFF); // X[32:12]
   return;
 }

1619–1621

ditto for break

1623

not sure why "Relocation" is uppercase.
I would write "Unsupported relocation for TLS GD to LE relaxation"

test/ELF/Inputs/aarch64-tls-ie.s

12

Why you do remove it in this patch ? its just excessive .text declaration it seems. I think it is not relative to the patch and should be fixed separatelly.

test/ELF/aarch64-tls-gdie.s

8

Not an issue actually, but we often put the REQUIRES to be the first line of test. That looks better IMO

11

Add a space here please "# Global-Dynamic..."

[zatrazz]

I will update a new patch with fixes for your comments.

ELF/Target.cpp

1590

Yeap, I will fix it.

1598

Ok.

1602

Ok.

1614

Ok.

1619–1621

Ok.

1623

Right, and the wording is wrong, it should be "TLS GD to IE". I will change that.

test/ELF/Inputs/aarch64-tls-ie.s

12

I just noted it is non required .text declaration and since it being used on the testcase being added to create the shared library I decided to cleanup this up. I can let as it and clean before or after this patch.

test/ELF/aarch64-tls-gdie.s

8

Right, I will change that.

11

Ok.

[zatrazz]

Updated patch based on previous comments.

[ruiu]

ELF/Target.cpp

1367

Please run clang-format-diff on this patch.

[zatrazz]

Fix format issues.

[zatrazz]

Ping.

[zatrazz]

Ping.

[grimar]

You abandoned this too.
Mistake ?

ELF/Target.cpp

void relocateOne(uint8_t *Loc, uint8_t *BufEnd, uint32_t Type, uint64_t P,
                    uint8_t *PairedLoc = nullptr) const override;
   size_t relaxTls(uint8_t *Loc, uint8_t *BufEnd, uint32_t Type, uint64_t P,
                   uint64_t SA, const SymbolBody *S) const override;
   bool refersToGotEntry(uint32_t Type) const override;
 
 private:
   void relocateTlsGdToLe(uint32_t Type, uint8_t *Loc, uint8_t *BufEnd,
                          uint64_t P, uint64_t SA) const;
+  void relocateTlsGdToIe(uint32_t Type, uint8_t *Loc, uint8_t *BufEnd,
+                         uint64_t P, uint64_t SA) const;
   void relocateTlsIeToLe(uint32_t Type, uint8_t *Loc, uint8_t *BufEnd,
                          uint64_t P, uint64_t SA) const;
 
   static const uint64_t TcbSize = 16;
 };
 
 class AMDGPUTargetInfo final : public TargetInfo {
 public:
default:
     return false;
   }
 }
 
 bool AArch64TargetInfo::needsGot(uint32_t Type, SymbolBody &S) const {
   if (Type == R_AARCH64_TLSIE_LD64_GOTTPREL_LO12_NC ||
       Type == R_AARCH64_TLSIE_ADR_GOTTPREL_PAGE21)
     return true;
+  if (isTlsDescRel(Type, S))
+    return canBePreempted(&S);
   return refersToGotEntry(Type) || needsPlt<ELF64LE>(Type, S);
 }
 
 bool AArch64TargetInfo::needsPltImpl(uint32_t Type) const {
   switch (Type) {
   default:
     return false;
   case R_AARCH64_CALL26:

 
 size_t AArch64TargetInfo::relaxTls(uint8_t *Loc, uint8_t *BufEnd, uint32_t Type,
                                    uint64_t P, uint64_t SA,
                                    const SymbolBody *S) const {
   switch (Type) {
   case R_AARCH64_TLSDESC_ADR_PAGE21:
   case R_AARCH64_TLSDESC_LD64_LO12_NC:
   case R_AARCH64_TLSDESC_ADD_LO12_NC:
-  case R_AARCH64_TLSDESC_CALL: {
+  case R_AARCH64_TLSDESC_CALL:
     if (canBePreempted(S))
-      fatal("Unsupported TLS optimization");
-    uint64_t X = S ? S->getVA<ELF64LE>() : SA;
-    relocateTlsGdToLe(Type, Loc, BufEnd, P, X);
+      relocateTlsGdToIe(Type, Loc, BufEnd, P, SA);
+    else
+      relocateTlsGdToLe(Type, Loc, BufEnd, P, SA);
     return 0;
-  }
   case R_AARCH64_TLSIE_ADR_GOTTPREL_PAGE21:
   case R_AARCH64_TLSIE_LD64_GOTTPREL_LO12_NC:
     relocateTlsIeToLe(Type, Loc, BufEnd, P, S->getVA<ELF64LE>());
     return 0;
   }
   llvm_unreachable("Unknown TLS optimization");
 }
 
case R_AARCH64_TLSDESC_LD64_LO12_NC:
     NewInst = 0xf2800000 | ((X & 0xffff) << 5);
     break;
   default:
     llvm_unreachable("Unsupported Relocation for TLS GD to LE relax");
   }
   write32le(Loc, NewInst);
 }
 
+// Global-Dynamic relocations can be relaxed to Initial-Exec if both binary is
+// an executable and target is not final (can be preempted).
+void AArch64TargetInfo::relocateTlsGdToIe(uint32_t Type, uint8_t *Loc,
+                                          uint8_t *BufEnd, uint64_t P,
+                                          uint64_t SA) const {
+  // TLSDESC Global-Dynamic relocation are in the form:
+  //   adrp    x0, :tlsdesc:v             [R_AARCH64_TLSDESC_ADR_PAGE21]
+  //   ldr     x1, [x0, #:tlsdesc_lo12:v] [R_AARCH64_TLSDESC_LD64_LO12_NC]
+  //   add     x0, x0, :tlsdesc_los:v     [R_AARCH64_TLSDESC_ADD_LO12_NC]
+  //   .tlsdesccall                       [R_AARCH64_TLSDESC_CALL]
+  // And it can optimized to:
+  //   adrp    x0, :tlsie:v
+  //   ldr     x0, [x0, :tlsie_lo12:v]
+  //   nop
+  //   nop
+
+  // For Initial-Exec from Global-Dynamic TLS relax the idea is to emit
+  // a R_AARCH64_TLS_TPREL64 dynamic relocation and thus adjust the
+  // tlsdesc call to access the GOT slot adjusted by dynamic linker.
+  checkUInt<32>(SA, Type);
+
+  switch (Type) {
+  case R_AARCH64_TLSDESC_ADD_LO12_NC:
+  case R_AARCH64_TLSDESC_CALL:
+    // nop
+    write32le(Loc, 0xD503201F);
+    break;
+  case R_AARCH64_TLSDESC_ADR_PAGE21: {
+    // adrp
+    uint64_t X = getAArch64Page(SA) - getAArch64Page(P);
+    updateAArch64Addr(Loc, (X >> 12) & 0x1FFFFF); // X[32:12]
+    return;
+  }
+  case R_AARCH64_TLSDESC_LD64_LO12_NC: {
+    // ldr with target register being 'x0'
+    uint32_t NewInst = read32le(Loc) & 0xFFFFFFE0;
+    NewInst |= (SA & 0xFF8) << 7;
+    write32le(Loc, NewInst);
+    break;
+  }
+  default:
+    llvm_unreachable("Unsupported relocation for TLS GD to IE relaxation");
+  }
+}
+
 // Initial-Exec relocations can be relaxed to Local-Exec if symbol is final
 // (can not be preempted).
 void AArch64TargetInfo::relocateTlsIeToLe(uint32_t Type, uint8_t *Loc,
                                           uint8_t *BufEnd, uint64_t P,
                                           uint64_t SA) const {
   uint64_t TPOff = llvm::alignTo(TcbSize, Out<ELF64LE>::TlsPhdr->p_align);
   uint64_t X = SA + TPOff;
   checkUInt<32>(X, Type);

test/ELF/Inputs/aarch64-tls-ie.s

-.text
  .global foo
  .section .tdata,"awT",%progbits
  .align 2
  .type foo, %object
  .size foo, 4
 foo:
  .word 5
  .text
 
-.text
  .global bar
  .section .tdata,"awT",%progbits
  .align 2
  .type bar, %object
  .size bar, 4
 bar:
  .word 5
  .text

test/ELF/aarch64-tls-gdie.s

+# REQUIRES: aarch64
+# RUN: llvm-mc -filetype=obj -triple=aarch64-unknown-linux %p/Inputs/aarch64-tls-ie.s -o %ttlsie.o
+# RUN: llvm-mc -filetype=obj -triple=aarch64-unknown-linux %s -o %tmain.o
+# RUN: ld.lld -shared %ttlsie.o -o %t.so
+# RUN: ld.lld %tmain.o -o %tout %t.so
+# RUN: llvm-objdump -d %tout | FileCheck %s
+# RUN: llvm-readobj -s -r %tout | FileCheck -check-prefix=RELOC %s
+
+# Global-Dynamic to Initial-Exec relax creates a R_AARCH64_TPREL64
+# RELOC:      Relocations [
+# RELOC-NEXT:   Section ({{.*}}) .rela.dyn {
+# RELOC-NEXT:    {{.*}} R_AARCH64_TLS_TPREL64 foo 0x0
+# RELOC-NEXT:   }
+# RELOC-NEXT: ]
+
+# CHECK: Disassembly of section .text:
+# CHECK: _start:
+# CHECK:  11000:  00 00 00 b0 adrp    x0, #4096
+# CHECK:  11004:  00 58 40 f9   ldr     x0, [x0, #176]
+# CHECK:  11008:  1f 20 03 d5   nop
+# CHECK:  1100c:  1f 20 03 d5   nop
+
+.text
+.globl _start
+_start:
+ adrp    x0, :tlsdesc:foo
+ ldr     x1, [x0, :tlsdesc_lo12:foo]
+ add     x0, x0, :tlsdesc_lo12:foo
+ .tlsdesccall foo
+ blr     x1