[llvm-dev] Usage of the jumptable attribute
CompilerCrash via llvm-dev
llvm-dev at lists.llvm.org
Wed Sep 4 10:09:27 PDT 2019
Hi Krzysztof,
thank you for your reply. That explains a lot.
I wonder if it is possible to let llvm create plt entries for
user-written functions as if they were dynamic library functions. Does
anyone know?
Best
Daniel
Am 04.09.2019 um 15:08 schrieb Krzysztof Parzyszek:
> Hi Daniel,
> This attribute doesn't do anything anymore. It was added a while back to implement control flow integrity, but that work was eventually removed. I believe it's still there for backwards compatibility (i.e. to read old bitcode).
>
> --
> Krzysztof Parzyszek kparzysz at quicinc.com AI tools development
>
>> -----Original Message-----
>> From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of CompilerCrash
>> via llvm-dev
>> Sent: Saturday, August 31, 2019 2:22 AM
>> To: llvm-dev at lists.llvm.org
>> Subject: [EXT] [llvm-dev] Usage of the jumptable attribute
>>
>> Hello everyone,
>>
>> I'm new to LLVM (which is a really great project by the way) and I hope this
>> is the right place for my question.
>>
>> In the LLVM Language Reference Manual I found the "jumptable" function
>> attribute, which seems to be exactly what I need for my project (where I want
>> to add one level of indirection to every function call), but I have trouble
>> figuring out, how to use it correctly.
>>
>> I wrote my own transformation pass (using LLVM 8.0.1) to add this attribute
>> as well as unnamed_addr (I'm aware that this might break some programs, but
>> jumptable requires it) to every function definition:
>>
>> // includes ...
>>
>> using namespace llvm;
>>
>> namespace {
>> struct MyPass : public ModulePass {
>> static char ID;
>> MyPass() : ModulePass(ID) {}
>>
>> bool runOnModule(Module &M) override {
>> for (Function &F : M.getFunctionList()) {
>> if (!F.isDeclaration()) {
>> F.setUnnamedAddr(GlobalValue::UnnamedAddr::Global);
>> F.addFnAttr(Attribute::JumpTable);
>> }
>> }
>> return true;
>> }
>> };
>> }
>>
>> char MyPass::ID = 0;
>> static RegisterPass<MyPass> X("mypass", "My Pass");
>>
>> And I wrote a program test.c:
>>
>> #include <stdio.h>
>>
>> void foo() {
>> puts("Hello World");
>> }
>>
>> int main() {
>> foo();
>> }
>>
>> I compiled it to test.ll and ran it through opt:
>>
>> clang -O0 -emit-llvm -S -o test.ll test.c
>> build/bin/opt -load build/lib/LLVMMyPass.so -mypass -S -o test.out.ll
>> test.ll
>>
>> The output (test.out.ll) was identical to test.ll except for the 3
>> occurrences of jumptable and the last 2 occurrences of unnamed_addr:
>>
>> ; ModuleID = 'test.ll'
>> source_filename = "test.c"
>> target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
>> target triple = "x86_64-unknown-linux-gnu"
>>
>> @.str = private unnamed_addr constant [12 x i8] c"Hello World\00", align
>> 1
>>
>> ; Function Attrs: jumptable noinline nounwind optnone uwtable
>> define dso_local void @foo() unnamed_addr #0 {
>> %1 = call i32 @puts(i8* getelementptr inbounds ([12 x i8], [12 x
>> i8]* @.str, i32 0, i32 0))
>> ret void
>> }
>>
>> declare dso_local i32 @puts(i8*) #1
>>
>> ; Function Attrs: jumptable noinline nounwind optnone uwtable
>> define dso_local i32 @main() unnamed_addr #0 {
>> call void @foo()
>> ret i32 0
>> }
>>
>> attributes #0 = { jumptable noinline nounwind optnone uwtable "no-jump-
>> tables"="false"
>> ; ... many more attributes ...
>> }
>> attributes #1 = {
>> ; ... many more attributes ...
>> }
>>
>> !llvm.module.flags = !{!0}
>> !llvm.ident = !{!1}
>>
>> !0 = !{i32 1, !"wchar_size", i32 4}
>> !1 = !{!"clang version 8.0.0 (tags/RELEASE_800/final)"}
>>
>> Finally, I executed:
>>
>> build/bin/llc -o test.out.s test.out.ll
>> clang -o test.out.native test.out.s
>> objdump --disassemble-all test.out.native
>>
>> I hoped to see some kind of jump table being used for the call to foo (maybe
>> even the plt mechanism the dynamic library calls use), but it is just a
>> "normal" jump to foo's address:
>>
>> 0000000000400500 <foo>:
>> 400500: 55 push %rbp
>> 400501: 48 89 e5 mov %rsp,%rbp
>> 400504: 48 bf c0 05 40 00 00 movabs $0x4005c0,%rdi
>> 40050b: 00 00 00
>> 40050e: e8 cd fe ff ff callq 4003e0 <puts at plt>
>> 400513: 5d pop %rbp
>> 400514: c3 retq
>> 400515: 66 2e 0f 1f 84 00 00 nopw %cs:0x0(%rax,%rax,1)
>> 40051c: 00 00 00
>> 40051f: 90 nop
>>
>> 0000000000400520 <main>:
>> 400520: 55 push %rbp
>> 400521: 48 89 e5 mov %rsp,%rbp
>> 400524: e8 d7 ff ff ff callq 400500 <foo>
>> 400529: 31 c0 xor %eax,%eax
>> 40052b: 5d pop %rbp
>> 40052c: c3 retq
>> 40052d: 0f 1f 00 nopl (%rax)
>>
>> So, what am I doing wrong? The documentation indicates that the jump table is
>> created at code-generation time. So I guess my pass works fine and I just
>> need to get the code generator to process the jumptable attribute somehow? I
>> found the createJumpInstrTablesPass in include/CodeGen/Passes.h, but it seems
>> like it's never used.
>>
>> Thanks in advance
>>
>> Daniel
>> _______________________________________________
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>> llvm-dev at lists.llvm.org
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