[llvm-dev] Usage of the jumptable attribute

[Krzysztof Parzyszek via llvm-dev]

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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