[llvm-dev] Mapping InlineAsm parameters to ConstraintInfoVector elements (original) (raw)

Alexander Potapenko via llvm-dev llvm-dev at lists.llvm.org
Mon Oct 29 05:14:48 PDT 2018

To close the loop, mapping inputs/outputs to constraints isn't actually that complicated.

Constraints in ConstraintInfoVector are grouped by type; the first nO constraints are outputs, they're followed by nI inputs and nC clobbers. The type is a member of InlineAsm::ConstraintInfo. A CallInst calling an InlineAsm has an optional return value, which is either a scalar, or a structure containing nV outputs returnable by value. The remaining nO-nV first operands of that CallInst are its outputs returned by pointer. Other nI operands are the inputs. The total number of InlineAsm operands is therefore nO-nV+nI+1 (the last one is the InlineAsm instance). On Mon, Mar 19, 2018 at 3:16 PM Alexander Potapenko <glider at google.com> wrote:

Sorry, I've stated the problem incorrectly. Turns out that I'm seeing false positives exactly because MSan fails to see that the inline assembly routine initializes its outputs. _E.g. for functions like testandsetbit() in the kernel, there's a local variable holding the return value: _static _alwaysinline bool testandsetbit(long nr, volatile unsigned long *addr) { bool oldbit; asm("bts %2,%1" CCSET(c) : CCOUT(c) (oldbit), ADDR : "Ir" (nr)); return oldbit; } , which the tool treats as uninitialized, unless we explicitly set it to false before calling asm().

On Fri, Mar 16, 2018 at 11:43 PM Evgenii Stepanov <eugeni.stepanov at gmail.com> wrote:

Could you provide an example where MSan checks an output parameter? On Fri, Mar 16, 2018 at 9:53 AM, Alexander Potapenko via llvm-dev <llvm-dev at lists.llvm.org> wrote: > Hi all, > > I'm trying to figure out which parameters of a given InlineAsm instruction > are its inputs, and which are the outputs (rationale: make sure MSan > doesn't check the output parameters of an asm() statement). > > As far as I understand, this information is only available through the > ConstraintInfoVector for the InlineAsm. However there's no exact match > between the constraints and the InlineAsm params. In addition to clobber > constraints, which should be safe to ignore, there can be multiple > occurrences of a single parameter having different constraints. Also, for > InlineAsm instructions returning non-void values there's one additional > constraint, but it's unclear whether it always goes first. > > My current implementation is quite ad-hoc, so I'm wondering if there's any > code that already does what I'm trying to achieve. > > Thanks in advance, > Alexander > > -- > Alexander Potapenko > Software Engineer > > Google Germany GmbH > Erika-Mann-Straße, 33 > 80636 München > > Geschäftsführer: Paul Manicle, Halimah DeLaine Prado > Registergericht und -nummer: Hamburg, HRB 86891 > Sitz der Gesellschaft: Hamburg _> ________________________ > LLVM Developers mailing list > llvm-dev at lists.llvm.org > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev -- Alexander Potapenko Software Engineer Google Germany GmbH Erika-Mann-Straße, 33 80636 München Geschäftsführer: Paul Manicle, Halimah DeLaine Prado Registergericht und -nummer: Hamburg, HRB 86891 Sitz der Gesellschaft: Hamburg

-- Alexander Potapenko Software Engineer

Google Germany GmbH Erika-Mann-Straße, 33 80636 München

Geschäftsführer: Paul Manicle, Halimah DeLaine Prado Registergericht und -nummer: Hamburg, HRB 86891 Sitz der Gesellschaft: Hamburg

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