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[lld][ARM] don't use short thumb thunks if no branch range extension
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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
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@stuij
stuij committed Jan 9, 2023
1 parent 1f11d1b commit 747fc27
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10 changes: 7 additions & 3 deletions lld/ELF/Thunks.cpp
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Expand Up @@ -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) {
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149 changes: 149 additions & 0 deletions lld/test/ELF/arm-thumb-range-thunk-os-no-ext.s
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@@ -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

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