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[LLD][ELF] Allow merging XO and RX sections, and add --[no-]xosegment flag (#132412)
Following from the discussion in #132224, this seems like the best approach to deal with a mix of XO and RX output sections in the same binary. This change will also simplify the implementation of the PURECODE section flag for AArch64. To control this behaviour, the `--[no-]xosegment` flag is added to LLD (similarly to `--[no-]rosegment`), which determines whether to allow merging XO and RX sections in the same segment. The default value is `--no-xosegment`, which is a breaking change compared to the previous behaviour. Release notes are also added, since this will be a breaking change.
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‎lld/ELF/Config.h

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@@ -341,6 +341,7 @@ struct Config {
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llvm::DenseSet<llvm::StringRef> saveTempsArgs;
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llvm::SmallVector<std::pair<llvm::GlobPattern, uint32_t>, 0> shuffleSections;
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bool singleRoRx;
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bool singleXoRx;
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bool shared;
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bool symbolic;
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bool isStatic = false;

‎lld/ELF/Driver.cpp

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@@ -1485,6 +1485,7 @@ static void readConfigs(Ctx &ctx, opt::InputArgList &args) {
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ctx.arg.randomizeSectionPadding =
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args::getInteger(args, OPT_randomize_section_padding, 0);
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ctx.arg.singleRoRx = !args.hasFlag(OPT_rosegment, OPT_no_rosegment, true);
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ctx.arg.singleXoRx = !args.hasFlag(OPT_xosegment, OPT_no_xosegment, false);
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ctx.arg.soName = args.getLastArgValue(OPT_soname);
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ctx.arg.sortSection = getSortSection(ctx, args);
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ctx.arg.splitStackAdjustSize =

‎lld/ELF/Options.td

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@@ -432,6 +432,10 @@ defm rosegment: BB<"rosegment",
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"Put read-only non-executable sections in their own segment (default)",
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"Do not put read-only non-executable sections in their own segment">;
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defm xosegment: BB<"xosegment",
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"Put execute-only sections in their own segment",
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"Do not put execute-only sections in their own segment (default)">;
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defm rpath: Eq<"rpath", "Add a DT_RUNPATH to the output">;
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def relocatable: F<"relocatable">, HelpText<"Create relocatable object file">;

‎lld/ELF/Writer.cpp

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@@ -2379,10 +2379,16 @@ Writer<ELFT>::createPhdrs(Partition &part) {
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// so when hasSectionsCommand, since we cannot introduce the extra alignment
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// needed to create a new LOAD)
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uint64_t newFlags = computeFlags(ctx, sec->getPhdrFlags());
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// When --no-rosegment is specified, RO and RX sections are compatible.
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uint32_t incompatible = flags ^ newFlags;
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if (ctx.arg.singleRoRx && !(newFlags & PF_W))
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incompatible &= ~PF_X;
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uint64_t incompatible = flags ^ newFlags;
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if (!(newFlags & PF_W)) {
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// When --no-rosegment is specified, RO and RX sections are compatible.
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if (ctx.arg.singleRoRx)
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incompatible &= ~PF_X;
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// When --no-xosegment is specified (the default), XO and RX sections are
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// compatible.
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if (ctx.arg.singleXoRx)
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incompatible &= ~PF_R;
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}
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if (incompatible)
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load = nullptr;
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‎lld/docs/ReleaseNotes.rst

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@@ -42,8 +42,16 @@ ELF Improvements
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* Linker script ``OVERLAY`` descriptions now support virtual memory regions
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(e.g. ``>region``) and ``NOCROSSREFS``.
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* Added ``--xosegment`` and ``--no-xosegment`` flags to control whether to place
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executable-only and readable-executable sections in the same segment. The
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default value is ``--no-xosegment``.
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(`#132412 <https://github.com/llvm/llvm-project/pull/132412>`_)
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Breaking changes
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----------------
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* Executable-only and readable-executable sections are now allowed to be placed
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in the same segment by default. Pass ``--xosegment`` to lld in order to get
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the old behavior back.
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COFF Improvements
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-----------------

‎lld/test/ELF/aarch64-execute-only-mixed.s

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// REQUIRES: aarch64
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// RUN: rm -rf %t && split-file %s %t && cd %t
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// RUN: llvm-mc -filetype=obj -triple=aarch64 start.s -o start.o
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// RUN: llvm-mc -filetype=obj -triple=aarch64 xo.s -o xo.o
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// RUN: llvm-mc -filetype=obj -triple=aarch64 rx.s -o rx.o
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// RUN: ld.lld start.o xo.o -o xo
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// RUN: ld.lld start.o rx.o -o rx-default
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// RUN: ld.lld --xosegment start.o rx.o -o rx-xosegment
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// RUN: ld.lld --no-xosegment start.o rx.o -o rx-no-xosegment
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// RUN: llvm-readelf -l xo | FileCheck --check-prefix=CHECK-XO %s
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// RUN: llvm-readelf -l rx-default | FileCheck --check-prefix=CHECK-MERGED %s
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// RUN: llvm-readelf -l rx-xosegment | FileCheck --check-prefix=CHECK-SEPARATE %s
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// RUN: llvm-readelf -l rx-no-xosegment | FileCheck --check-prefix=CHECK-MERGED %s
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// CHECK-XO: PHDR
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// CHECK-XO-NEXT: LOAD
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// CHECK-XO-NEXT: LOAD 0x000120 0x0000000000210120 0x0000000000210120 0x00000c 0x00000c E 0x10000
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/// Index should match the index of the LOAD segment above.
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// CHECK-XO: 02 .text .foo
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// CHECK-MERGED: PHDR
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// CHECK-MERGED-NEXT: LOAD
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// CHECK-MERGED-NEXT: LOAD 0x000120 0x0000000000210120 0x0000000000210120 0x00000c 0x00000c R E 0x10000
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/// Index should match the index of the LOAD segment above.
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// CHECK-MERGED: 02 .text .foo
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// CHECK-SEPARATE: PHDR
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// CHECK-SEPARATE-NEXT: LOAD
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// CHECK-SEPARATE-NEXT: LOAD 0x000158 0x0000000000210158 0x0000000000210158 0x000008 0x000008 E 0x10000
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// CHECK-SEPARATE-NEXT: LOAD 0x000160 0x0000000000220160 0x0000000000220160 0x000004 0x000004 R E 0x10000
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/// Index should match the index of the LOAD segment above.
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// CHECK-SEPARATE: 02 .text
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// CHECK-SEPARATE: 03 .foo
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//--- start.s
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.section .text,"axy",@progbits,unique,0
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.global _start
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_start:
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bl foo
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ret
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//--- xo.s
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.section .foo,"axy",@progbits,unique,0
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.global foo
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foo:
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ret
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//--- rx.s
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/// Ensure that the implicitly-created .text section has the SHF_AARCH64_PURECODE flag.
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.section .text,"axy",@progbits,unique,0
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.section .foo,"ax",@progbits,unique,0
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.global foo
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foo:
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ret

‎lld/test/ELF/aarch64-execute-only.s

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@@ -1,12 +1,12 @@
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// REQUIRES: aarch64
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// RUN: llvm-mc -filetype=obj -triple=aarch64 %s -o %t.o
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// RUN: ld.lld %t.o -o %t.so -shared
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// RUN: ld.lld --xosegment %t.o -o %t.so -shared
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// RUN: llvm-readelf -l %t.so | FileCheck --implicit-check-not=LOAD %s
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// RUN: echo ".section .foo,\"ax\"; ret" > %t.s
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// RUN: llvm-mc -filetype=obj -triple=aarch64 %t.s -o %t2.o
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// RUN: ld.lld %t.o %t2.o -o %t.so -shared
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// RUN: ld.lld --xosegment %t.o %t2.o -o %t.so -shared
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// RUN: llvm-readelf -l %t.so | FileCheck --check-prefix=DIFF --implicit-check-not=LOAD %s
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// CHECK: LOAD 0x000000 0x0000000000000000 0x0000000000000000 0x000245 0x000245 R 0x10000

‎lld/test/ELF/arm-execute-only-mixed.s

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// REQUIRES: arm
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// RUN: rm -rf %t && split-file %s %t && cd %t
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// RUN: llvm-mc -filetype=obj -triple=armv7 start.s -o start.o
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// RUN: llvm-mc -filetype=obj -triple=armv7 xo.s -o xo.o
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// RUN: llvm-mc -filetype=obj -triple=armv7 rx.s -o rx.o
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// RUN: ld.lld start.o xo.o -o xo
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// RUN: ld.lld start.o rx.o -o rx-default
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// RUN: ld.lld --xosegment start.o rx.o -o rx-xosegment
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// RUN: ld.lld --no-xosegment start.o rx.o -o rx-no-xosegment
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// RUN: llvm-readelf -l xo | FileCheck --check-prefix=CHECK-XO %s
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// RUN: llvm-readelf -l rx-default | FileCheck --check-prefix=CHECK-MERGED %s
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// RUN: llvm-readelf -l rx-xosegment | FileCheck --check-prefix=CHECK-SEPARATE %s
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// RUN: llvm-readelf -l rx-no-xosegment | FileCheck --check-prefix=CHECK-MERGED %s
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// CHECK-XO: PHDR
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// CHECK-XO-NEXT: LOAD
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// CHECK-XO-NEXT: LOAD 0x0000b4 0x000200b4 0x000200b4 0x0000c 0x0000c E 0x10000
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/// Index should match the index of the LOAD segment above.
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// CHECK-XO: 02 .text .foo
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// CHECK-MERGED: PHDR
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// CHECK-MERGED-NEXT: LOAD
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// CHECK-MERGED-NEXT: LOAD 0x0000b4 0x000200b4 0x000200b4 0x0000c 0x0000c R E 0x10000
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/// Index should match the index of the LOAD segment above.
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// CHECK-MERGED: 02 .text .foo
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// CHECK-SEPARATE: PHDR
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// CHECK-SEPARATE-NEXT: LOAD
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// CHECK-SEPARATE-NEXT: LOAD 0x0000d4 0x000200d4 0x000200d4 0x00008 0x00008 E 0x10000
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// CHECK-SEPARATE-NEXT: LOAD 0x0000dc 0x000300dc 0x000300dc 0x00004 0x00004 R E 0x10000
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/// Index should match the index of the LOAD segment above.
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// CHECK-SEPARATE: 02 .text
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// CHECK-SEPARATE: 03 .foo
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//--- start.s
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.section .text,"axy",%progbits,unique,0
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.global _start
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_start:
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bl foo
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bx lr
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//--- xo.s
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.section .foo,"axy",%progbits,unique,0
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.global foo
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foo:
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bx lr
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//--- rx.s
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/// Ensure that the implicitly-created .text section has the SHF_ARM_PURECODE flag.
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.section .text,"axy",%progbits,unique,0
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.section .foo,"ax",%progbits,unique,0
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.global foo
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foo:
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bx lr

‎lld/test/ELF/arm-execute-only.s

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@@ -1,13 +1,13 @@
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// REQUIRES: arm
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// RUN: llvm-mc -filetype=obj -triple=armv7-pc-linux %s -o %t.o
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// RUN: ld.lld %t.o -o %t.so -shared
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// RUN: ld.lld --xosegment %t.o -o %t.so -shared
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// RUN: llvm-readelf -l %t.so | FileCheck --implicit-check-not=LOAD %s
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// RUN: echo ".section .foo,\"ax\"; \
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// RUN: bx lr" > %t.s
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// RUN: llvm-mc -filetype=obj -triple=armv7-pc-linux %t.s -o %t2.o
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// RUN: ld.lld %t.o %t2.o -o %t.so -shared
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// RUN: ld.lld --xosegment %t.o %t2.o -o %t.so -shared
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// RUN: llvm-readelf -l %t.so | FileCheck --check-prefix=DIFF --implicit-check-not=LOAD %s
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// CHECK: LOAD 0x000000 0x00000000 0x00000000 0x0016d 0x0016d R 0x10000

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