[llvm-dev] Immediate value boundary checking

[Sky Flyer via llvm-dev]

Hi Alex,

That helped a lot. I appreciate it. :-)

On Mon, Dec 7, 2015 at 1:24 PM, Alex Bradbury <asb at asbradbury.org> wrote:

> On 7 December 2015 at 11:51, Sky Flyer via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> > Dear all,
> >
> > I have written an assembler which reads assembly instructions and
> produces
> > the equivalent binary. I have a problem. Although I set the bit range and
> > immediate type for an instruction like add which accepts a register and
> an
> > immediate value, I can simply overflow that value and llvm/tablegen
> doesn't
> > care!
> >
> > for example for a i8imm imm value (bits<8> val) these two produce the
> same
> > output:
> >
> > add r0 0
> > add r0 256
> >
> > whose responsibility is to check the integer boundary in the assembly
> > parser?
>
> It's worth looking at what some other targets do here (e.g. SystemZ
> handles this pretty cleanly, and MIPS is adding better error reporting
> http://reviews.llvm.org/D15226).
>
> It helps to take a look at include/llvm/Target/Target.td. the Operand
> class has a field ParserMatchClass of type AsmOperandClass, which by
> default contains the ImmAsmOperand instance which ultimately means
> that isImm() in your AsmParser will be called to check the operand. To
> customise the behaviour to for instance, check the immediate is in the
> appropriate range you can provide your own AsmOperandClass. e.g.:
>
> class ImmediateAsmOperand<string name>
>   : AsmOperandClass {
>   let Name = name;
>   let RenderMethod = "addImmOperands";
>   let DiagnosticType = !strconcat("Invalid", name);
> }
>
> def imm8 : Operand<i32> {
>   let ParserMatchClass = ImmediateAsmOperand<"Imm8">;
> }
>
> With the Name field of the ParserMatchClass set to Imm8, You now just
> need to provide an isImm8() implementation in MyTargetAsmParser.
> You'll also want to check for Match_InvalidImm8 as a result of
> MatchInstructionImpl to provide a more useful error reporting than
> "invalid operand for instruction". To use Match_InvalidImm8, you'll
> need to make sure you do something like the below (just grep the
> sources for GET_OPERAND_DIAGNOSTIC_TYPES to see other targets doing
> the same).
>
>   enum MyTargetMatchResultTy {
>     Match_Dummy = FIRST_TARGET_MATCH_RESULT_TY,
> #define GET_OPERAND_DIAGNOSTIC_TYPES
> #include "MyTargetGenAsmMatcher.inc"
> #undef GET_OPERAND_DIAGNOSTIC_TYPES
>   };
>
> I hope that helps.
>
> Best,
>
> Alex
>
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