[llvm-dev] BUGS in code generated for target i386-win32

[Reid Kleckner via llvm-dev]

It's worth pointing out that we *do* add EAX as an implicit output of every
asm block, since some MSVC inline asm does stuff like this in SDK headers
and we have to support it:
  int foo(int x) {
    __asm mov eax, x
  }

There's a bug for this somewhere in bugzilla with more details, but I'm
late to work and wasn't able to find it in my first search.

We could similarly detect __asm blocks at the start of a function, and add
implicit inputs for all parameters, and that would get the desired
behavior, with inlining to boot, which MSVC doesn't get. It's worth filing
a bug about it in any case.

As Tim says, if you just want clang to leave this code alone, add an
explicit return and put __declspec(naked) on the function. That'll turn off
all the parameter IPO as well.

Adding warnings for this is out of scope. If we took the time to analyze
the user's usage of registers and correlate it with the target calling
convention, we might as well take the time to implement this wacky behavior.

On Mon, Nov 26, 2018 at 10:24 AM Tim Northover via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi Stefan,
>
> On Mon, 26 Nov 2018 at 18:01, Stefan Kanthak <stefan.kanthak at nexgo.de>
> wrote:
> > > I'll try to explain a little below how that one mismatch causes the
> > > issues you're seeing.
> > >
> > >> BUG #1: the compiler fails to allocate (EAX for) the variable "lfsr"!
> > >> BUG #2: the variable "lfsr" is NOT initialized!
> > >
> > > Since the __asm isn't linked (as far as Clang is concerned) to
> > > either input for lfsr32, they're both unused.
> >
> > REALLY? Or better: OUCH!
> > Is Clang NOT aware of the __fastcall calling convention and its
> > register usage?
>
> It is, but it intentionally does not tie that knowledge together with
> any __asm blocks, even if they are the only statement in a function
> body.
>
> > Clang does NOT create prolog/epilog here, so it CLEARLY knows that
> > "argument" is held in ECX and "polynomial" in EDX ... as documented
> > for __fastcall
>
> That doesn't follow from the lack of a prologue, though it happens to
> be true. There is still no link with the register usage in the __asm
> however.
>
> > JFTR: Clang (like MSVC too) knows VERY well which registers are used
> >       (clobbered) in an __asm block. See <https://godbolt.org/z/blDIzK>,
> >       which proves your assumption WRONG!
>
> I don't believe so. Clobbering a register is very different from
> guessing some input value to put into that register prior to the
> block. For MSVC you're apparently supposed to write inline asm that
> materializes values into registers from elsewhere (variables of one
> kind or another) before using them.
>
> I could see the behaviour you're expecting being a reasonable default
> for a __declspec(naked) function restricted to a single __asm block,
> but it interacts very poorly with C language semantics for functions
> that can contain both C and assembly.
>
> > 2. add "__declspec(noinline)" to the function definition ... and notice
> >    that the compiler STILL does NOT setup ANY of EAX, ECX and EDX before
> >    the function call!
> >    See <https://godbolt.org/z/X9R9w7>, which again proves your
> assumption
> >    WRONG.
>
> The noinline prevents actual inlining, but the same inferences can be
> (and are) made through inter-procedural analysis. Add "-emit-llvm -g0"
> and you'll see your function being called with "undef" arguments and
> the complete elimination of lfsr.
>
> > If the function is declared only, but not defined, then the compiler
> > sets the arguments up before the call: see <https://godbolt.org/z/4pmOlC
> >
>
> Indeed.
>
> > >> additionally it holds the second argument with the __fastcall calling
> > >> convention, so the constant 0x04C11DB7 MUST be loaded into EDX before
> > >> label LBB1_1:
> > >
> > > This is the same unused-as-far-as-the-compiler-knows thing as the
> first two.
> >
> > OUCH!
> > <https://godbolt.org/z/X9R9w7> proves this WRONG.
>
> I'm afraid I don't see how. It's an unused variable (as far as Clang
> is concerned) so it doesn't need to be materialized anywhere.
>
> > My expectation is that AT LEAST a warning message should be printed by
> > the compiler.
>
> For using a register before defining it, perhaps? That's within the
> implementable realm I think (at least to cover some cases), though
> whether anyone is interested enough to actually do it is another
> question.
>
> Cheers.
>
> Tim.
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