[PATCH] Add an intrinsic for creating jump-instruction table entries

[Eric Christopher]

Seems interesting, haven't looked too much, but I'm in favor of
anything that stops people from writing inline asm from passes :)

Nick? Chris?

-eric

On Thu, Mar 20, 2014 at 4:15 PM, Tom Roeder <tmroeder at google.com> wrote:
> On Thu, Mar 20, 2014 at 2:37 PM, Tom Roeder <tmroeder at google.com> wrote:
>>
>> Hi,
>>
>> This patch adds an intrinsic for creating jump-instruction table entries
>> of the form:
>>
>>   .type f_JT, at function
>>   f_JT:
>>     jmp f at PLT
>>
>> It adds an intrinsic void @llvm.jump.instr.table.entry(i8*) and X86
>> backend support for this intrinsic. The intrinsic takes a bitcast function
>> pointer as input and generates the jump-instruction table entry. Note that
>> while jump tables are supported in CodeGen, they are a different kind of
>> jump table, AFAICT: one that uses indices rather than one that contains jump
>> instructions.
>>
>> This intrinsic would allow me to remove the inline assembly that I
>> currently use for control-flow integrity (CFI), as discussed on llvmdev;
>> these intrinsics would be generated by the CFI pass along with declarations
>> of the jump-table functions (required because the CFI pass replaces
>> address-taken functions with the address of the _JT function).
>>
>> I couldn't find any way to do this above the arch-specific Targets, given
>> that it violates the IR branch semantics of jumping to a block; and even so,
>> I had to add a check to one of the X86 optimizations that was assuming that
>> all JMP_4 instructions had a MachineBasicBlock as an operand.
>>
>> The patch also adds another kind of label, JUMP_FUNCTION_LABEL, which
>> produces a label that looks like the start of a function (with MCSA_Global
>> and sometimes MCSA_ELF_TypeFunction). So, the intrinsic lowers to a
>> JUMP_FUNCTION_LABEL and a JMP_4.
>>
>> I was able to do much of the work in the target-independent portions of
>> the code, so I don't think it will take much effort to add support to some
>> other architectures (like ARM), and I intend to do that if this patch is
>> acceptable.
>>
>> One question: what's the right way to do the mangling (the "_JT") for the
>> jump-table functions? For my purposes in CFI, I'd like to have a way of
>> naming these jump entries that a pass at the IR level could also discover or
>> reproduce. Is there a standard mangling facility for specialized suffixes?
>>
>> Please let me know what you think.
>>
>> Thanks,
>>
>> Tom
>
>
> Just realized that there were a few line-length violations that I hadn't
> cleaned up in the code. Here's a slightly tweaked patch.
>
> Tom
>
>
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