[lld][ARM] don't use short thumb thunks if no branch range extension

In ThumbThunk::isCompatibleWith, we check if we can use short thunks if we are
within branch range. However these short thumb thunks will generate b.w
instructions, and these are not available on pre branch range extension
architectures.

On these architectures (v4, v5, and most of v6), we could replace the b.w with a
Thumb b (2) instruction, but that would in an ideal situation only give us an
extra range of 2048 bytes on top of the 4MB range of a BL, if a thunk section
happens to be placed on the outer range of a BL and the stars are aligned. It
doesn't seem worth it.

What would be worth it is a state change to Arm and a subsequent branch to
either Arm or Thumb code. But that's the subject of another patch.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D140633

diff --git a/lld/ELF/Thunks.cpp b/lld/ELF/Thunks.cpp
index d805dee9ac4b72e97216f5103a315d2af99baad2..52c0ffc28ab114fb9e39ddd2d0e98f2a8ec8abe4 100644 (file)
--- a/lld/ELF/Thunks.cpp
+++ b/lld/ELF/Thunks.cpp
@@ -542,10 +542,14 @@ bool ARMThunk::isCompatibleWith(const InputSection &isec,
   return rel.type != R_ARM_THM_JUMP19 && rel.type != R_ARM_THM_JUMP24;
 }
 
-// This function returns true if the target is Thumb and is within 2^25, and
-// it has not previously returned false (see comment for mayUseShortThunk).
+// This function returns true if:
+// the target is Thumb
+// && is within branch range
+// && this function has not previously returned false
+//    (see comment for mayUseShortThunk)
+// && the arch supports Thumb branch range extension.
 bool ThumbThunk::getMayUseShortThunk() {
-  if (!mayUseShortThunk)
+  if (!mayUseShortThunk || !config->armJ1J2BranchEncoding)
     return false;
   uint64_t s = getARMThunkDestVA(destination);
   if ((s & 1) == 0) {

diff --git a/lld/test/ELF/arm-thumb-range-thunk-os-no-ext.s b/lld/test/ELF/arm-thumb-range-thunk-os-no-ext.s
new file mode 100644 (file)
index 0000000..1f511e1
--- /dev/null
+++ b/lld/test/ELF/arm-thumb-range-thunk-os-no-ext.s
@@ -0,0 +1,149 @@
+// REQUIRES: arm
+// RUN: llvm-mc -arm-add-build-attributes -filetype=obj -triple=thumbv4t-none-linux-gnueabi %s -o %t.o
+// RUN: ld.lld %t.o -o %t2
+/// The output file is large, most of it zeroes. We dissassemble only the
+/// parts we need to speed up the test and avoid a large output file.
+// RUN: llvm-objdump --no-show-raw-insn -d %t2 --start-address=0x100000 --stop-address=0x10000c | FileCheck --check-prefix=CHECK1 %s
+// RUN: llvm-objdump --no-show-raw-insn -d %t2 --start-address=0x200000 --stop-address=0x200002 | FileCheck --check-prefix=CHECK2 %s
+// RUN: llvm-objdump --no-show-raw-insn -d %t2 --start-address=0x300000 --stop-address=0x300002 | FileCheck --check-prefix=CHECK3 %s
+// RUN: llvm-objdump --no-show-raw-insn -d %t2 --start-address=0x400000 --stop-address=0x400028 | FileCheck --check-prefix=CHECK4 %s
+// RUN: llvm-objdump --no-show-raw-insn -d %t2 --start-address=0x700000 --stop-address=0x700014 | FileCheck --check-prefix=CHECK5 %s
+// RUN: llvm-objdump --no-show-raw-insn -d %t2 --start-address=0x800000 --stop-address=0x800004 | FileCheck --check-prefix=CHECK6 %s
+// RUN: llvm-objdump --no-show-raw-insn -d %t2 --start-address=0x900000 --stop-address=0x900004 | FileCheck --check-prefix=CHECK7 %s
+// RUN: llvm-objdump --no-show-raw-insn -d %t2 --start-address=0xa00000 --stop-address=0xa00018 | FileCheck --check-prefix=CHECK8 %s
+// RUN: llvm-objdump --no-show-raw-insn -d %t2 --start-address=0xb00000 --stop-address=0xb00004 | FileCheck --check-prefix=CHECK9 %s
+
+/// Test the Range extension Thunks for Thumb when all the code is in a single
+/// OutputSection. The Thumb BL instruction has a range of 4Mb. We create a
+/// series of functions a megabyte apart. We expect range extension thunks to be
+/// created when a branch is out of range. Thunks will be reused whenever they
+/// are in range.
+.syntax unified
+
+/// Define a function aligned on a megabyte boundary.
+.macro FUNCTION suff
+  .section .text.\suff\(), "ax", %progbits
+  .thumb
+  .balign 0x100000
+  .globl tfunc\suff\()
+  .type  tfunc\suff\(), %function
+  tfunc\suff\():
+  bx lr
+.endm
+
+.section .text, "ax", %progbits
+.thumb
+.globl _start
+_start:
+  /// tfunc00 and tfunc03 are within 4MB, so no Range Thunks expected.
+  bl tfunc00
+  bl tfunc03
+  /// tfunc04 is > 4MB away, expect a Range Thunk to be generated, to go into
+  /// the first of the pre-created ThunkSections.
+  bl tfunc04
+
+// CHECK1-LABEL: <_start>:
+// CHECK1-NEXT:   100000:       bl      0x200000 <tfunc00>      @ imm = #0xffffc
+// CHECK1-NEXT:                 bl      0x500000 <tfunc03>      @ imm = #0x3ffff8
+// CHECK1-NEXT:                 bl      0x400008 <__Thumbv4ABSLongThunk_tfunc04> @ imm = #0x2ffffc
+
+FUNCTION 00
+// CHECK2-LABEL: <tfunc00>:
+// CHECK2-NEXT:   200000:       bx      lr
+
+FUNCTION 01
+// CHECK3-LABEL: <tfunc01>:
+// CHECK3-NEXT:   300000:       bx      lr
+
+FUNCTION 02
+  bl tfunc07
+  /// The thunks should not generate a v7-style short thunk.
+// CHECK4-LABEL: <tfunc02>:
+// CHECK4-NEXT:   400000:       bx      lr
+// CHECK4-NEXT:                 bl      0x400018 <__Thumbv4ABSLongThunk_tfunc07> @ imm = #0x12
+// CHECK4-NEXT:                 bmi     0x3fffb2 <tfunc01+0xfffb2> @ imm = #-0x58
+// CHECK4-EMPTY:
+// CHECK4-NEXT:  <__Thumbv4ABSLongThunk_tfunc04>:
+// CHECK4-NEXT:   400008:              bx      pc
+// CHECK4-NEXT:                 b       0x400008 <__Thumbv4ABSLongThunk_tfunc04> @ imm = #-0x6
+// CHECK4-EMPTY:
+// CHECK4-NEXT: <$a>:
+// CHECK4-NEXT:  40000c:        ldr        r12, [pc]               @ 0x400014 <__Thumbv4ABSLongThunk_tfunc04+0xc>
+// CHECK4-NEXT:                 bx         r12
+// CHECK4-EMPTY:
+// CHECK4-NEXT:  <$d>:
+// CHECK4-NEXT:   400014: 01 00 60 00          .word   0x00600001
+// CHECK4-EMPTY:
+// CHECK4-NEXT:  <__Thumbv4ABSLongThunk_tfunc07>:
+// CHECK4-NEXT:   400018:              bx          pc
+// CHECK4-NEXT:                b           0x400018 <__Thumbv4ABSLongThunk_tfunc07> @ imm = #-0x6
+// CHECK4-EMPTY:
+// CHECK4-NEXT:  <$a>:
+// CHECK4-NEXT:   40001c:              ldr     r12, [pc]                   @ 0x400024 <__Thumbv4ABSLongThunk_tfunc07+0xc>
+// CHECK4-NEXT:                bx      r12
+// CHECK4-EMPTY:
+// CHECK4-NEXT:  <$d>:
+// CHECK4-NEXT:   400024: 01 00 90 00          .word   0x00900001
+
+FUNCTION 03
+FUNCTION 04
+FUNCTION 05
+// CHECK5-LABEL: <tfunc05>:
+// CHECK5-NEXT:   700000:              bx      lr
+// CHECK5-NEXT:                bmi     0x6fffae <tfunc04+0xfffae> @ imm = #-0x58
+// CHECK5-EMPTY:
+// CHECK5-NEXT:  <__Thumbv4ABSLongThunk_tfunc03>:
+// CHECK5-NEXT:   700004:              bx      pc
+// CHECK5-NEXT:                b       0x700004 <__Thumbv4ABSLongThunk_tfunc03> @ imm = #-0x6
+// CHECK5-EMPTY:
+// CHECK5-NEXT:  <$a>:
+// CHECK5-NEXT:   700008:              ldr     r12, [pc]               @ 0x700010 <__Thumbv4ABSLongThunk_tfunc03+0xc>
+// CHECK5-NEXT:                bx      r12
+// CHECK5-EMPTY:
+// CHECK5-NEXT:  <$d>:
+// CHECK5-NEXT:   700010: 01 00 50 00          .word   0x00500001
+
+FUNCTION 06
+  /// The backwards branch is within range, so no range extension necessary.
+  bl tfunc04
+// CHECK6-LABEL: <tfunc06>:
+// CHECK6-NEXT:   800000:              bx      lr
+// CHECK6-NEXT:                    bl  0x600000 <tfunc04>      @ imm = #-0x200006
+
+FUNCTION 07
+  /// The backwards branch is out of range.
+  bl tfunc03
+// CHECK7-LABEL: <tfunc07>:
+// CHECK7-NEXT:   900000:              bx      lr
+// CHECK7-NEXT:                bl      0x700004 <__Thumbv4ABSLongThunk_tfunc03> @ imm = #-0x200002
+
+FUNCTION 08
+  /// 2nd backwards branch outside of range to same fn. Should share thunk with
+  /// previous call.
+  bl tfunc03
+// CHECK8-LABEL: <tfunc08>:
+// CHECK8-NEXT:   a00000:              bx      lr
+// CHECK8-NEXT:                bl      0x700004 <__Thumbv4ABSLongThunk_tfunc03> @ imm = #-0x300002
+// CHECK8-NEXT:                bmi     0x9fffb2 <tfunc07+0xfffb2> @ imm = #-0x58
+// CHECK8-EMPTY:
+// CHECK8-NEXT:  <__Thumbv4ABSLongThunk_tfunc03>:
+// CHECK8-NEXT:   a00008:              bx      pc
+// CHECK8-NEXT:   a0000a:              b       0xa00008 <__Thumbv4ABSLongThunk_tfunc03> @ imm = #-0x6
+// CHECK8-EMPTY:
+// CHECK8-NEXT:  <$a>:
+// CHECK8-NEXT:   a0000c:              ldr     r12, [pc]               @ 0xa00014 <__Thumbv4ABSLongThunk_tfunc03+0xc>
+// CHECK8-NEXT:   a00010:              bx      r12
+// CHECK8-EMPTY:
+// CHECK8-NEXT:  <$d>:
+// CHECK8-NEXT:   a00014: 01 00 50 00          .word   0x00500001
+
+FUNCTION 09
+  /// This call is out of range of ThunkSection at 0700004.
+  /// These 3 calls to tfunc03 could have used the same thunk (section), but
+  /// we are not that sophisticated.
+  bl tfunc03
+// CHECK9-LABEL: <tfunc09>:
+// CHECK9-NEXT:   b00000:              bx      lr
+// CHECK9-NEXT:                bl      0xa00008 <__Thumbv4ABSLongThunk_tfunc03> @ imm = #-0xffffe
+FUNCTION 10
+FUNCTION 11