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

Tom Roeder tmroeder at google.com
Thu Mar 20 14:37:51 PDT 2014


This patch adds an intrinsic for creating jump-instruction table entries of
the form:

  .type f_JT, at function
    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

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

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

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

Please let me know what you think.


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