[PPC64] Define getThunkSectionSpacing() based on the range of R_PPC64_REL24

[MaskRay]

Summary

Suggested by Sean Fertile and Peter Smith.

Thunk section spacing decrease the total number of thunks. I measured a
decrease of 1% or less in some large programs, with no perceivable
slowdown in link time. Override getThunkSectionSpacing() to enable it.
0x2000000 is the farthest point R_PPC64_REL24 can reach. I tried several
numbers and found 0x2000000 works the best. Numbers near 0x2000000 work
as well but let's just use the simpler number.

As demonstrated by the updated tests, this essentially changes placement
of most thunks to the end of the output section. We leverage this
property to fix PR40740 reported by Alfredo Dal'Ava Júnior:

The output section .init consists of input sections from several object
files (crti.o crtbegin.o crtend.o crtn.o). Sections other than the last
one do not have a terminator. With this patch, we create the thunk after
the last .init input section and thus fix the issue. This is not
foolproof but works quite well for such sections (with no terminator) in
practice.

[MaskRay]

Number of thunks with difference choices of getThunkSectionSpacing:

A

0: 79692
0x2000000-0x10000: 78449
0x2000000-0xc000: 78336
0x2000000-0x8000: 78332
0x2000000-0x4000: 78386
0x2000000-0x2000: 78365
0x2000000-0x1000: 78345
0x2000000-0x0000: 78320
0x2000000+0x4000: 78334

B

0: 112315
0x2000000-0xc000: 112474
0x2000000-0x8000: 112333
0x2000000-0x4000: 111937
0x2000000-0x0000: 111838
0x2000000+0x4000: 111851
0x2000000+0x8000: 111847

The "simplest" number 0x2000000 works no worse than other numbers so I'll just use it.

[MaskRay]

Update description

[sfertile]

The "simplest" number 0x2000000 works no worse than other numbers so I'll just use it.

That is pretty reasonable. LGTM.

test/ELF/ppc64-call-reach.s

68 ↗

(On Diff #198779)

This comment is out of date and can be removed.

[MaskRay]

Delete a comment

[MaskRay]

I've checked this against our huge internal code base. This doesn't cause new failures.

[ruiu]

LGTM

[MaskRay]

Update descriptions

lld/trunk/ELF/Arch/PPC64.cpp

RelExpr getRelExpr(RelType Type, const Symbol &S,
                      const uint8_t *Loc) const override;
   void writePltHeader(uint8_t *Buf) const override;
   void writePlt(uint8_t *Buf, uint64_t GotPltEntryAddr, uint64_t PltEntryAddr,
                 int32_t Index, unsigned RelOff) const override;
   void relocateOne(uint8_t *Loc, RelType Type, uint64_t Val) const override;
   void writeGotHeader(uint8_t *Buf) const override;
   bool needsThunk(RelExpr Expr, RelType Type, const InputFile *File,
                   uint64_t BranchAddr, const Symbol &S) const override;
+  uint32_t getThunkSectionSpacing() const override;
   bool inBranchRange(RelType Type, uint64_t Src, uint64_t Dst) const override;
   RelExpr adjustRelaxExpr(RelType Type, const uint8_t *Data,
                           RelExpr Expr) const override;
   void relaxGot(uint8_t *Loc, RelType Type, uint64_t Val) const override;
   void relaxTlsGdToIe(uint8_t *Loc, RelType Type, uint64_t Val) const override;
   void relaxTlsGdToLe(uint8_t *Loc, RelType Type, uint64_t Val) const override;
   void relaxTlsLdToLe(uint8_t *Loc, RelType Type, uint64_t Val) const override;
   void relaxTlsIeToLe(uint8_t *Loc, RelType Type, uint64_t Val) const override;
bool PPC64::needsThunk(RelExpr Expr, RelType Type, const InputFile *File,
   // If the offset exceeds the range of the branch type then it will need
   // a range-extending thunk.
   // See the comment in getRelocTargetVA() about R_PPC64_CALL.
   return !inBranchRange(Type, BranchAddr,
                         S.getVA() +
                             getPPC64GlobalEntryToLocalEntryOffset(S.StOther));
 }
 
+uint32_t PPC64::getThunkSectionSpacing() const {
+  // See comment in Arch/ARM.cpp for a more detailed explanation of
+  // getThunkSectionSpacing(). For PPC64 we pick the constant here based on
+  // R_PPC64_REL24, which is used by unconditional branch instructions.
+  // 0x2000000 = (1 << 24-1) * 4
+  return 0x2000000;
+}
+
 bool PPC64::inBranchRange(RelType Type, uint64_t Src, uint64_t Dst) const {
   int64_t Offset = Dst - Src;
   if (Type == R_PPC64_REL14)
     return isInt<16>(Offset);
   if (Type == R_PPC64_REL24)
     return isInt<26>(Offset);
   llvm_unreachable("unsupported relocation type used in branch");
 }

lld/trunk/test/ELF/ppc64-bsymbolic-toc-restore.s

 # Note that the bl .+44 is a call to def's local entry, jumping past the first 2
 # instructions. Branching to the global entry would corrupt the TOC pointer
 # since the global entry requires that %r12 hold the address of the function
 # being called.
 
 # CHECK-LABEL: caller
 # CHECK:         bl .+44
 # CHECK-NEXT:    mr 31, 3
-# CHECK-NEXT:    bl .-48
+# CHECK-NEXT:    bl .+44
 # CHECK-NEXT:    ld 2, 24(1)
 # CHECK-NEXT:    add 3, 3, 31
 # CHECK-NEXT:    addi 1, 1, 32
 # CHECK-NEXT:    ld 0, -16(1)
 # CHECK-NEXT:    mtlr 0
 # CHECK-NEXT:    blr
 # CHECK-EMPTY:
 # CHECK-NEXT:  def:
 # CHECK-NEXT:    addis 2, 12, 2
-# CHECK-NEXT:    addi 2, 2, -32636
+# CHECK-NEXT:    addi 2, 2, -32616
 # CHECK-NEXT:    li 3, 55
 # CHECK-NEXT:    blr

lld/trunk/test/ELF/ppc64-call-reach.s

 # CHECK-LABEL: test
 # CHECK:  10010014:       bl .+33554428
 # CHECK:  10010024:       b  .+33554428
 
 # NEGOFFSET-LABEL: test
 # NEGOFFSET:  10010014:       bl .-33554432
 # NEGOFFSET:  10010024:       b  .+33554432
 
+# THUNK-LABEL: test:
+# THUNK: 10010014:       bl .+20
+# THUNK: 10010024:       b .+20
+
 # .branch_lt[0]
 # THUNK-LABEL: __long_branch_callee:
-# THUNK-NEXT: 10010000:       addis 12, 2, 1
+# THUNK-NEXT: 10010028:       addis 12, 2, 1
 # THUNK-NEXT:                 ld 12, -32768(12)
 # THUNK-NEXT:                 mtctr 12
 # THUNK-NEXT:                 bctr
 
 # .branch_lt[1]
 # THUNK-LABEL: __long_branch_tail_callee:
-# THUNK-NEXT: 10010010:       addis 12, 2, 1
+# THUNK-NEXT: 10010038:       addis 12, 2, 1
 # THUNK-NEXT:                 ld 12, -32760(12)
 # THUNK-NEXT:                 mtctr 12
 # THUNK-NEXT:                 bctr
 
-# Each call now branches to a thunk, and although it is printed as positive
-# the offset is interpreted as a signed 26 bit value so 67108812 is actually
-# -52.
-# THUNK-LABEL: test:
-# THUNK: 10010034:       bl .-52
-# THUNK: 10010044:       b .+67108812
-
 # The offset from the TOC to the .branch_lt section  is (-1 << 16) - 32768.
 #                Name             Type            Address          Off    Size
 # BRANCHLT:     .got              PROGBITS        0000000010020000 020000 000008
 # BRANCHLT:     .branch_lt        PROGBITS        0000000010030000 030000 000010
 # BRANCHLT-NOT: .plt

lld/trunk/test/ELF/ppc64-ifunc.s

 # RUN: llvm-mc -filetype=obj -triple=powerpc64-unknown-linux %s -o %t.o
 # RUN: ld.lld %t.o -o %t
 # RUN: llvm-nm %t | FileCheck --check-prefix=NM %s
 # RUN: llvm-readelf -S %t | FileCheck --check-prefix=SECTIONS %s
 # RUN: llvm-objdump -d --no-show-raw-insn %t | FileCheck %s
 # RUN: llvm-readelf -r %t | FileCheck --check-prefix=DYNREL %s
 
 # NM-DAG: 0000000010028000 d .TOC.
-# NM-DAG: 0000000010010028 T ifunc
-# NM-DAG: 000000001001002c T ifunc2
+# NM-DAG: 0000000010010000 T ifunc
+# NM-DAG: 0000000010010004 T ifunc2
 
 # SECTIONS: .plt NOBITS 0000000010030000
 
+# __plt_ifunc - . = 0x10010020 - 0x10010010 = 16
+# __plt_ifunc2 - . = 0x10010044 - 0x10010018 = 28
+# CHECK: _start:
+# CHECK-NEXT:                 addis 2, 12, 1
+# CHECK-NEXT:                 addi 2, 2, 32760
+# CHECK-NEXT: 10010010:       bl .+16
+# CHECK-NEXT:                 ld 2, 24(1)
+# CHECK-NEXT: 10010018:       bl .+28
+# CHECK-NEXT:                 ld 2, 24(1)
+
 # .plt[0] - .TOC. = 0x10030000 - 0x10028000 = (1<<16) - 32768
 # CHECK: __plt_ifunc:
 # CHECK-NEXT:     std 2, 24(1)
 # CHECK-NEXT:     addis 12, 2, 1
 # CHECK-NEXT:     ld 12, -32768(12)
 # CHECK-NEXT:     mtctr 12
 # CHECK-NEXT:     bctr
 
 # .plt[1] - .TOC. = 0x10030000+8 - 0x10028000 = (1<<16) - 32760
 # CHECK: __plt_ifunc2:
 # CHECK-NEXT:     std 2, 24(1)
 # CHECK-NEXT:     addis 12, 2, 1
 # CHECK-NEXT:     ld 12, -32760(12)
 # CHECK-NEXT:     mtctr 12
 # CHECK-NEXT:     bctr
 
-# __plt_ifunc - . = 0x10010000 - 0x10010038 = -56
-# __plt_ifunc2 - . = 0x10010014 - 0x10010040 = -44
-# CHECK: _start:
-# CHECK-NEXT:                 addis 2, 12, 1
-# CHECK-NEXT:                 addi 2, 2, 32720
-# CHECK-NEXT: 10010038:       bl .-56
-# CHECK-NEXT:                 ld 2, 24(1)
-# CHECK-NEXT: 10010040:       bl .-44
-# CHECK-NEXT:                 ld 2, 24(1)
-
 # Check that we emit 2 R_PPC64_IRELATIVE.
-# DYNREL: R_PPC64_IRELATIVE       10010028
-# DYNREL: R_PPC64_IRELATIVE       1001002c
+# DYNREL: R_PPC64_IRELATIVE       10010000
+# DYNREL: R_PPC64_IRELATIVE       10010004
 
 .type ifunc STT_GNU_IFUNC
 .globl ifunc
 ifunc:
   nop
 
 .type ifunc2 STT_GNU_IFUNC
 .globl ifunc2

lld/trunk/test/ELF/ppc64-local-dynamic.s

 // When calculating offset relative to the dynamic thread pointer we have to
 // adjust by 0x8000 since each DTV pointer points 0x8000 bytes past the start of
 // its TLS block.
 // #ha(i@dtprel) --> (0x0 -0x8000 + 0x8000) >> 16 = 0
 // #lo(i@dtprel) --> (0x0 -0x8000) = -0x8000 = -32768
 // Dis:     test:
 // Dis:        addis 3, 2, 0
 // Dis-NEXT:   addi 3, 3, -32760
-// Dis-NEXT:   bl .-60
+// Dis-NEXT:   bl .+60
 // Dis-NEXT:   ld 2, 24(1)
 // Dis-NEXT:   addis 3, 3, 0
 // Dis-NEXT:   lwa 3, -32768(3)
 
 
 // #hi(j@got@tlsld) --> (0x20108 - 0x28100 ) > 16 = -1
 // Dis: test_hi:
 // Dis:   lis 3, -1
 
 // k@got@tlsld --> (0x20108 - 0x28100) = -7ff8 = -32760
 // Dis: test_16:
 // Dis:   li 3, -32760

lld/trunk/test/ELF/ppc64-long-branch-init.s

+# REQUIRES: ppc
+
+# RUN: llvm-mc -filetype=obj -triple=powerpc64-pc-freebsd13.0 %s -o %t.o
+# RUN: ld.lld %t.o -o %t
+# RUN: llvm-objdump -d --no-show-raw-insn %t | FileCheck %s
+
+## .init consists of sections from several object files. Sections other than the
+## last one do not have a terminator. Check we do not create a long branch stub
+## in the middle.
+## We currently use thunk section spacing to ensure the stub is in the end. This
+## is not foolproof but good enough to not break in practice.
+
+# CHECK: Disassembly of section .init:
+# CHECK-EMPTY:
+# CHECK-LABEL: _init:
+# CHECK:         blr
+# CHECK-EMPTY:
+# CHECK-LABEL: __long_branch_foo:
+
+.globl foo
+foo:
+  .space 0x2000000
+  blr
+
+.section .init,"ax",@progbits,unique,0
+.globl _init
+_init:
+  stdu 1, -48(1)
+  mflr 0
+  std 0, 64(1)
+
+.section .init,"ax",@progbits,unique,1
+  bl foo
+  nop
+
+.section .init,"ax",@progbits,unique,2
+  bl foo
+  nop
+
+.section .init,"ax",@progbits,unique,3
+  ld 1, 0(1)
+  ld 0, 16(1)
+  mtlr 0
+  blr

lld/trunk/test/ELF/ppc64-plt-stub.s

 // REQUIRES: ppc
 
 // RUN: llvm-mc -filetype=obj -triple=powerpc64le-unknown-linux %s -o %t.o
 // RUN: llvm-mc -filetype=obj -triple=powerpc64le-unknown-linux %p/Inputs/shared-ppc64.s -o %t2.o
 // RUN: ld.lld -shared %t2.o -o %t2.so
 // RUN: ld.lld %t.o %t2.so -o %t
-// RUN: llvm-objdump -d %t | FileCheck %s
+// RUN: llvm-objdump -d --no-show-raw-insn %t | FileCheck %s
 
 // RUN: llvm-mc -filetype=obj -triple=powerpc64-unknown-linux %s -o %t.o
 // RUN: llvm-mc -filetype=obj -triple=powerpc64-unknown-linux %p/Inputs/shared-ppc64.s -o %t2.o
 // RUN: ld.lld -shared %t2.o -o %t2.so
 // RUN: ld.lld %t.o %t2.so -o %t
-// RUN: llvm-objdump -d %t | FileCheck %s
+// RUN: llvm-objdump -d --no-show-raw-insn %t | FileCheck %s
 
 // CHECK:      Disassembly of section .text:
 // CHECK-EMPTY:
-// CHECK-NEXT: __plt_foo:
+// CHECK-NEXT: _start:
+// CHECK:      10010008: bl .+16
+
+// CHECK-LABEL: 0000000010010018 __plt_foo:
 // CHECK-NEXT:      std 2, 24(1)
 // CHECK-NEXT:      addis 12, 2, 0
 // CHECK-NEXT:      ld 12, 32560(12)
 // CHECK-NEXT:      mtctr 12
 // CHECK-NEXT:      bctr
 
 
-// CHECK:            _start:
-// CHECK:            bl .-40
         .text
         .abiversion 2
         .globl  _start
         .p2align        4
         .type   _start,@function
 _start:
 .Lfunc_begin0:
 .Lfunc_gep0:

lld/trunk/test/ELF/ppc64-toc-restore-recursive-call.s

 # REQUIRES: ppc
 
 # RUN: llvm-mc -filetype=obj -triple=powerpc64le-unknown-linux %s -o %t.o
 # RUN: ld.lld -shared %t.o -o %t.so
 # RUN: llvm-objdump -d --no-show-raw-insn -r %t.so | FileCheck %s
 
 # For a recursive call that is interposable the linker calls the plt-stub rather
 # then calling the function directly. Since the call is through a plt stub and
 # might be interposed with a different definition at runtime, a toc-restore is
 # required to follow the call.
 
 # The decision to use a plt-stub for the recursive call is not one I feel
 # strongly about either way. It was done because it matches what bfd and gold do
 # for recursive calls as well as keeps the logic for recursive calls consistent
 # with non-recursive calls.
 
-# CHECK-LABEL: __plt_recursive_func:
-# CHECK-NEXT: 10000:
-# CHECK-LABEL: recursive_func
-# CHECK-NEXT:  10014:
-# CHECK:       1003c:       bl .-60
-# CHECK-NEXT:  10040:       ld 2, 24(1)
+# CHECK-LABEL: 0000000000010000 recursive_func:
+# CHECK:       10028:       bl .+32
+# CHECK-NEXT:               ld 2, 24(1)
+
+# CHECK-LABEL: 0000000000010048 __plt_recursive_func:
 
         .abiversion 2
         .section ".text"
         .p2align 2
         .global recursive_func
         .type recursive_func, @function
 recursive_func:
 .Lrf_gep:

lld/trunk/test/ELF/ppc64-toc-restore.s

 
 # Calling external function foo in a shared object needs a nop.
 # Calling local function bar_local doe not need a nop.
 .global _start
 _start:
   bl foo
   nop
   bl bar_local
-
-
-// CHECK: Disassembly of section .text:
+// CHECK-LABEL: _start:
+// CHECK-NEXT:  10010008:       bl .+64
+// CHECK-NEXT:  1001000c:       ld 2, 24(1)
+// CHECK-NEXT:  10010010:       bl .-16
 // CHECK-EMPTY:
-// CHECK: _start:
-// CHECK:     1001001c:       bl .-28
-// CHECK-NOT: 10010020:       nop
-// CHECK:     10010020:       ld 2, 24(1)
-// CHECK:     10010024:       bl .-16
-// CHECK-NOT: 10010028:       nop
-// CHECK-NOT: 10010028:       ld 2, 24(1)
 
 # Calling a function in another object file which will have same
 # TOC base does not need a nop. If nop present, do not rewrite to
 # a toc restore
 .global diff_object
 _diff_object:
   bl foo_not_shared
   bl foo_not_shared
   nop
-
-// CHECK: _diff_object:
-// CHECK-NEXT: 10010028:       bl .+24
-// CHECK-NEXT: 1001002c:       bl .+20
-// CHECK-NEXT: 10010030:       nop
+// CHECK-LABEL: _diff_object:
+// CHECK-NEXT:  10010014:       bl .+28
+// CHECK-NEXT:  10010018:       bl .+24
+// CHECK-NEXT:  1001001c:       nop
 
 # Branching to a local function does not need a nop
 .global noretbranch
 noretbranch:
   b bar_local
-// CHECK: noretbranch:
-// CHECK:     10010034:        b .+67108832
-// CHECK-NOT: 10010038:        nop
-// CHECK-NOT: 1001003c:        ld 2, 24(1)
+// CHECK-LABEL: noretbranch:
+// CHECK:       10010020:        b .+67108832
+// CHECK-EMPTY:
 
 // This should come last to check the end-of-buffer condition.
 .global last
 last:
   bl foo
   nop
-// CHECK: last:
-// CHECK:      10010038:       bl .-56
-// CHECK-NEXT: 1001003c:       ld 2, 24(1)
+// CHECK-LABEL: last:
+// CHECK-NEXT:  10010024:       bl .+36
+// CHECK-NEXT:  10010028:       ld 2, 24(1)