x86: How to Force 2-byte `jmp` instruction in lowering

I have a bit of a riddle:

In http://reviews.llvm.org/D19904 I’m trying to spell the following assembly:

.palign 2, 0x90
jmp +0x9
nopw 512(%rax,%rax,1)
// rest of the code

I try the following snippet to accomplish this:

auto Target = OutContext.createLinkerPrivateTempSymbol();

// Use a two-byte jmp. This version of JMP takes an 8-bit relative offset as
// an operand (computed as an offset from the jmp instruction).

.addExpr(MCSymbolRefExpr::create(Target, OutContext)),
EmitNops(*OutStreamer, 9, Subtarget->is64Bit(), getSubtargetInfo());

Which turns into:

.palign 2, 0x90
jmp .Ltmp0
nopw 512(%rax,%rax,1)
// rest of the code

Is there a way of forcing the lowered JMP instruction to turn into a two-byte jump that does a short relative jump (one that fits within 8 bits)? When I run the binary and disassemble the function I’m seeing it turn into a 5-byte jump (jmpq <32-bit offset>) instead of a 2-byte jump (jmp <8-bit offset>).

Thanks in advance!

This appears to work:

auto Target = OutContext.createLinkerPrivateTempSymbol();


auto Target = OutContext.createTempSymbol();


Thanks Nirav – I tried this but I’m still getting a “jmpq ” with this incantation when I load and disassemble from gdb. I’m seeing a 5-instruction jump, followed by the nops.

If I disassemble with llvm-objdump though I see the following:

400c10: e9 09 00 00 00 jmp 9 <_Z3foov+0xE>
400c15: 66 0f 1f 84 00 00 02 00 00 nopw 512(%rax,%rax)

I’m not sure whether the extra 0’s after ‘0xe9 0x09’ are alignment padding (though I was expecing 0x90 to show up if this was an alignment issue).

Is there anything else I can try here?

Thanks in advance!

Hmm. Odd. I just rebuilt from scratch and it seems to work with the test/CodeGen/X86/xray-attribute-instrumentation.ll test case outputing straight to obj:

llc -filetype=obj -o ~/a.o -mtriple=x86_64-apple-macosx < test/CodeGen/X86/xray-attribute-instrumentation.ll

What test case are you using?

In any case, the issue appears to be that llvm doesn’t realize that the target address is resolved and erroneously applies branch relaxation to the jump. I don’t know why a linker private symbol would make a difference.


Thanks Nirav,

I can confirm that this works when I do the compile with llc, but then when linking to an executable with clang (patched with http://reviews.llvm.org/D20352 and compiler-rt patched with http://reviews.llvm.org/D21612) on Linux, I’m getting something different. Here’s a sample of the transcript, and what I’m seeing:

—>8 clang invocation 8<—

[16-06-23 3:33:42] dberris@dberris: ~/xray/llvm-build% ./bin/clang -fxray-instrument -x c++ -std=c++11 -o test.bin test.cc -g --verbose
clang version 3.9.0 (http://llvm.org/git/clang.git 3ae26ac8b1c9c5db65f3dc0236139448b8b0520a) (http://llvm.org/git/llvm.git 8fd5dd6aa8a633eeb03b245cd0060479371fc521)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /usr/local/google/home/dberris/xray/llvm-build/./bin
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.7
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.7.3
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.8
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.8.4
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.9
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.9.3
Selected GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.8
Candidate multilib: .;@m64
Candidate multilib: 32;@m32
Candidate multilib: x32;@mx32
Selected multilib: .;@m64
“/usr/local/google/home/dberris/xray/llvm-build/bin/clang-3.9” -cc1 -triple x86_64-unknown-linux-gnu -emit-obj -mrelax-all -disable-free -main-file-name test.cc -mrelocation-model static -mthread-model posix -mdisable-fp-elim -fmath-errno -masm-verbose -mconstructor-aliases -munwind-tables -fuse-init-array -target-cpu x86-64 -v -dwarf-column-info -debug-info-kind=limited -dwarf-version=4 -debugger-tuning=gdb -fxray-instrument -resource-dir /usr/local/google/home/dberris/xray/llvm-build/bin/…/lib/clang/3.9.0 -internal-isystem /usr/lib/gcc/x86_64-linux-gnu/4.8/…/…/…/…/include/c++/4.8 -internal-isystem /usr/lib/gcc/x86_64-linux-gnu/4.8/…/…/…/…/include/x86_64-linux-gnu/c++/4.8 -internal-isystem /usr/lib/gcc/x86_64-linux-gnu/4.8/…/…/…/…/include/x86_64-linux-gnu/c++/4.8 -internal-isystem /usr/lib/gcc/x86_64-linux-gnu/4.8/…/…/…/…/include/c++/4.8/backward -internal-isystem /usr/local/include -internal-isystem /usr/local/google/home/dberris/xray/llvm-build/bin/…/lib/clang/3.9.0/include -internal-externc-isystem /usr/include/x86_64-linux-gnu -internal-externc-isystem /include -internal-externc-isystem /usr/include -std=c++11 -fdeprecated-macro -fdebug-compilation-dir /usr/local/google/home/dberris/xray/llvm-build -ferror-limit 19 -fmessage-length 272 -fobjc-runtime=gcc -fcxx-exceptions -fexceptions -fdiagnostics-show-option -fcolor-diagnostics -o /tmp/test-03d46e.o -x c++ test.cc
clang -cc1 version 3.9.0 based upon LLVM 3.9.0svn default target x86_64-unknown-linux-gnu
ignoring nonexistent directory “/include”
ignoring duplicate directory “/usr/lib/gcc/x86_64-linux-gnu/4.8/…/…/…/…/include/x86_64-linux-gnu/c++/4.8”
#include “…” search starts here:
#include <…> search starts here:
End of search list.
“/usr/bin/ld” -z relro --hash-style=gnu --eh-frame-hdr -m elf_x86_64 -dynamic-linker /lib64/ld-linux-x86-64.so.2 -o test.bin /usr/lib/gcc/x86_64-linux-gnu/4.8/…/…/…/x86_64-linux-gnu/crt1.o /usr/lib/gcc/x86_64-linux-gnu/4.8/…/…/…/x86_64-linux-gnu/crti.o /usr/lib/gcc/x86_64-linux-gnu/4.8/crtbegin.o -L/usr/lib/gcc/x86_64-linux-gnu/4.8 -L/usr/lib/gcc/x86_64-linux-gnu/4.8/…/…/…/x86_64-linux-gnu -L/lib/x86_64-linux-gnu -L/lib/…/lib64 -L/usr/lib/x86_64-linux-gnu -L/usr/lib/gcc/x86_64-linux-gnu/4.8/…/…/… -L/usr/local/google/home/dberris/xray/llvm-build/bin/…/lib -L/lib -L/usr/lib -whole-archive /usr/local/google/home/dberris/xray/llvm-build/bin/…/lib/clang/3.9.0/lib/linux/libclang_rt.xray-x86_64.a -no-whole-archive /tmp/test-03d46e.o --no-as-needed -lpthread -lrt -lm -latomic -ldl -lgcc --as-needed -lgcc_s --no-as-needed -lc -lgcc --as-needed -lgcc_s --no-as-needed /usr/lib/gcc/x86_64-linux-gnu/4.8/crtend.o /usr/lib/gcc/x86_64-linux-gnu/4.8/…/…/…/x86_64-linux-gnu/crtn.o

—>8 clang invocation 8<—

The test.cc is simply:

—>8 test.cc 8<—

[[clang::xray_always_instrument]] void foo() { std::printf(“Hello, XRay!\n”); }

void bar() { std::printf(“Not instrumented\n”); }

extern “C” {
extern int __xray_patch();

int main(int argc, char* argv) {
printf(“main has started.\n”);
—>8 test.cc 8<—

A snippet of the disassembly (llvm-objdump -disassemble test.bin) looks like:

—>8 disassembly 8<—

400cb0: e9 09 00 00 00 jmp 9 <_Z3foov+0xE>
400cb5: 66 0f 1f 84 00 00 02 00 00 nopw 512(%rax,%rax)
400cbe: 55 pushq %rbp
400cbf: 48 89 e5 movq %rsp, %rbp
400cc2: 48 83 ec 10 subq $16, %rsp
400cc6: 48 bf c5 0e 40 00 00 00 00 00 movabsq $4198085, %rdi
400cd0: b0 00 movb $0, %al
400cd2: e8 a9 f9 ff ff callq -1623 <.plt+0x30>
400cd7: 89 45 fc movl %eax, -4(%rbp)
400cda: 48 83 c4 10 addq $16, %rsp
400cde: 5d popq %rbp
400cdf: c3 retq
400ce0: 2e 66 0f 1f 84 00 00 02 00 00 nopw %cs:512(%rax,%rax)
400cea: 66 0f 1f 44 00 00 nopw (%rax,%rax)

—>8 disassembly 8<—

Having looked at this a bit, I think you’re right that the jumps are being relaxed, due to the -mrelax-all option being used by clang. The question becomes whether it’s possible to inhibit relaxation for specific instructions at the LLVM level.


Peter suggested just writing out ‘.byte 0xeb, 0x09’ and that allowed the jump instruction to bypass the relaxation, so that fixes my immediate problem. The question still stands though whether it should be possible to do through the instruction builder interface.


I don't think so. When the relax-all flag is on MC will relax all instructions.


I see.

So the question becomes what’s the advantage (if any) of Clang passing the ‘relax-all’ flag down to MC? Is there a good reason for this behaviour at all?

Thanks Rafael!

Speed, but it has probably been years since anyone benchmarked that.


Okay. Any objections to just removing that from the clang side?

Depends on what the benchmarks show.


Relaxation is the process of *shortening* jumps that can be shortened, and
then re-running instruction layout to discover more relaxations until
fixpoint. Removing the -relax-all flag in clang won't help here, it would

I'm not exactly sure what the semantics of linker private symbols are, but
using a normal assembler temporary label is probably the way to go anyway.

I thought jumps start short and relaxation widens them as needed until fixpoint. So relax-all causes them all to be widened unconditionally.

My mistake, you're right.

I've been reading that code for years and assuming that it goes
large-to-small, but I guess the process is the same regardless of which
direction you go. :slight_smile:

Good point. Is there a set of benchmarks I can run to make sure that this is a mostly benign change? Something that comes with the test suite?


Okay, so then removing -mrelax-all should make this widening/shortening work more effectively? i.e. we wouldn’t be unconditionally widening short jumps and allowing the instruction layout process “work as intended”?