[llvm-dev] [RFC] Checking inline assembly for validity

Reid Kleckner via llvm-dev llvm-dev at lists.llvm.org
Mon Nov 26 12:36:14 PST 2018

I was following along until I got to the "way this works" part. :)

I agree with the premise 100%, writing correct inline assembly is a huge
pain. It would be nice to have some middle ground between having the user
carry the burden of writing correct constraints as in GCC inline asm, and
having the compiler guess the constraints as in MSVC inline asm.

The way I imagined this would work, before digging in too much, is that
we'd parse the GCC inline asm in the frontend, as is done with MS inline
asm, so you can get the warnings during semantic analysis. Generally,
GNU-as syntax and the GCC asm constraints should make this much, much
easier than it is for Intel syntax. The parser would parse "%N" as some new
kind of MCOperand. We'd write generic MC checks that walk the MC
instruction stream and leverage InstrInfo to produce warnings.

That's pretty far from what you had in mind. Does that seem crazy? I think
it would help clean up a lot of the ad-hoc inline asm constraint warning
logic we have in clang, which has tons of target-specific knowledge. It
also seems like a good first step towards a less string-ly typed inline asm
instruction in LLVM IR, which is perhaps a bit out of scope.

On Mon, Nov 26, 2018 at 2:54 AM Oliver Stannard <Oliver.Stannard at arm.com>

> GCC-style inline assembly is notoriously hard to write correctly, because
> it is
> the user's responsibility to tell the compiler about the requirements of
> the
> assembly (inputs, output, modified registers, memory access), and getting
> this
> wrong results in silently generating incorrect code. This is also
> dependent on
> register allocation and scheduling decisions made by the compiler, so an
> inline
> assembly statement may appear to work correctly, then silently break when
> another change to the code or compiler upgrade causes those decisions to
> change.
> I've posted a prototype patch at https://reviews.llvm.org/D54891 which
> tries to
> improve this situation by emitting diagnostics when the instructions
> inside the
> inline assembly string do not match the operands to the inline assembly
> statement. We can do this because we parse the assembly in the same
> process as
> the compiler, and the MC layer has some knowledge of which registers are
> read
> and written by an assembly instruction.
> For example, this C code, which tries to add 3 integers together, looks OK
> at
> first glance:
>   int add3(int a, int b, int c) {
>     int x;
>     asm volatile(
>         "add %0, %1, %2; add %0, %0, %3"
>       : "=r" (x)
>       : "r" (a), "r" (b), "r" (c));
>     return x;
>   }
> However, the compiler is allowed to allocate x (the output operand) and c
> (an
> input operand) to the same register, as it assumes that c will be read
> before x
> is written. This code might happen to work correctly when first written,
> but a
> later change (either in the source code or compiler) could cause register
> allocation to change, and this will start silently generating the "wrong"
> code.
> With this patch, the compiler emits this warning for the above code:
>   test.cpp:4:7: warning: read from an inline assembly operand after first
>                          output operand written, suggest adding
> early-clobber
>                          modifier to output operand [-Winline-asm]
>         "add %0, %1, %2; add %0, %0, %3"
>         ^
>   <inline asm>:1:30: note: instantiated into assembly here
>           add r0, r0, r1; add r0, r0, r2
>                                       ^
>   test.cpp:4:7: note: output operand written here
>         "add %0, %1, %2; add %0, %0, %3"
>         ^
>   <inline asm>:1:6: note: instantiated into assembly here
>           add r0, r0, r1; add r0, r0, r2
>               ^
> The warning is a bit noisy because it prints both the original source code
> and
> final assembly code, and the carets for the original source just point to
> the
> start of the line, but that's a separate issue.
> I've designed this to be independent of register allocation decisions, so
> the
> above diagnostic is always reported, regardless of whether x and c were
> allocated in the same register or not. This allows it to catch code which
> currently works but might break in future.
> The way this works is:
> - When the AsmParser reaches an INLINEASM MachineInstr, it creates an
>   InlineAsmDataflowChecker object, which tracks all of the information we
> need
>   to know about one inline assembly statement.
> - The AsmPrinter examines the operands to the MachineInstruction, and calls
>   functions on the tracking object to record information about each
> operand to
>   the inline assembly block, as provided by the user and relied on by the
>   compiler's optimisations and code-generation.
> - While the AsmParser is generating the final assembly string (which
> involves
>   expanding operand templates like "$0" into physical register names), it
>   records the offset from the start of the (output) string at which each
>   operand expansion appeared.
> - The table-generated assembly matcher is modified to record the index of
> the
>   MCParsedAsmOperand which resulted in in the creation of each MCOperand.
> An
>   MCParsedAsmOperand can create multiple MCOperands (for example, a memory
>   operand with base and offset), but not the other way round, so this
>   information is stored in the MCOperand.
> - When the AsmParser is running and a tracking object is present (it is
> only
>   present if we are parsing inline assembly), it records in each
> ParsedOperand
>   the indexes of the inline assembly statement operands which overlap with
> it.
>   This is done using the source location of the just-parsed
> MCParsedAsmOperand
>   and the string offsets recored in the tracking object by the AsmPrinter.
> - Finally, after an instruction has been completely parsed, the AsmParser
> calls
>   into the tracking object with the final MCInst and list of
>   MCParsedAsmOperands. With all of this information, we can match up inline
>   assembly operands to the MCOperands that were created for them, and check
>   that they match.
> The reason that I'm posting this as an RFC is that it adds a lot of
> coupling
> between different parts of the backend. Do people think this is an
> acceptable
> cost for making a quite user-hostile feature a bit safer? If people agree
> that
> this is worthwhile, there are some things I still need to do before the
> patch
> is ready for a proper review:
> - Check the memory overhead (during regular compilation) of storing the
> parsed
>   operand number in the MCOperand. If this is too high, it could be moved
> to a
>   separate data structure only created when parsing inline assmebly.
> - There are three different concepts of "operand" here (inline assembly
>   operand, MCParsedAsmOperand and MCOperand), tidy up the naming so that
> these
>   are a bit clearer.
> - For checks which depend on the order of instructions (for example,
> checking
>   that a clobbered register is only read from if it has previously been
> written
>   to), these could give false positives if there is control-flow in the
>   assembly. We could check if an MCInst affects control flow, buffer these
>   diagnostics until the end of the assembly block, and only emit them if
> there
>   are no control-flow instructions.
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