[llvm-dev] PR43374 - when should comparing NaN values raise a floating point exception?
Szabolcs Nagy via llvm-dev
llvm-dev at lists.llvm.org
Tue Oct 8 04:08:45 PDT 2019
* Sanjay Patel via llvm-dev <llvm-dev at lists.llvm.org> [2019-10-01 09:44:54 -0400]:
> Let's change the example to eliminate suspects:
> #include <math.h>
> int is_nan(float x) {
> /*
> The following subclauses provide macros that are quiet (non
> floating-point exception raising)
> versions of the relational operators, and other comparison macros
> that facilitate writing
> efficient code that accounts for NaNs without suffering the
> ‘‘invalid’’ floating-point exception.
> */
> return isunordered(x, x);
> }
>
> The comment text is from 7.12.14 of the C standard draft. I'm hoping to
> avoid any scenario under which it is ok to raise an exception in that code
> (eliminate any questions about the clang front-end behavior / FENV_ACCESS).
isunordered (like isnan) is a different operation from x!=x,
in particular it does not signal even on signaling nans
while x!=x does.
>
> As IR from clang with no optimization, this becomes a bunch of load/store
> with:
> %cmp = fcmp uno double %conv, %conv1
>
> Ok, so far? "fcmp uno" - http://llvm.org/docs/LangRef.html#fcmp-instruction
> :
> uno: yields true if either operand is a QNAN.
why is that ok?
here you need an unordered-not-equal, but got an unordered-compare,
the former is a silent ieee754 operation the latter is signaling.
as noted earlier this is wrong for signaling nans, because any fcmp
would signal for them, but the lack of snan support can be forgiven,
however here even qnans fail to remain silent, that's a bug.
>
> EarlyCSE/InstCombine reduce that fcmp to:
> %cmp = fcmp uno float %x, 0.000000e+00
>
> Still good? Same fcmp predicate, but we replaced a repeated use of "%x"
> with a zero constant to aid optimization.
no, this is different from even x!=x not to mention isunordered(x,x).
clang does not support FENV_ACCESS but repeatedly claimed in relevant
bug reports that users who care should use -O0, but it seems to create
broken compares even at -O0, so there is no way to get c99 conforming
behaviour.
>
> Now, send the optimized IR to codegen:
> define i32 @is_nan(float %x) {
> %cmp = fcmp uno float %x, 0.000000e+00
> %r = zext i1 %cmp to i32
> ret i32 %r
> }
>
> $ llc -o - fpexception.ll -mtriple=armv7a
> vmov s0, r0
> mov r0, #0
> vcmpe.f32 s0, s0
> vmrs APSR_nzcv, fpscr
> movwvs r0, #1
> bx lr
>
> We produced "vcmpe" for code that should never cause an FP exception. ARM
> codegen bug?
sorry, the arm code gen is right here, the bug is in clang.
>
> On Tue, Oct 1, 2019 at 5:45 AM Kristof Beyls <Kristof.Beyls at arm.com> wrote:
>
> > Hi,
> >
> > I’ve been investigating https://bugs.llvm.org/show_bug.cgi?id=43374,
> > which is about clang/llvm producing code that triggers a floating point
> > exception when x is NaN, when targeting ARM, in the below code example.
> >
> > int bar(float x) {
> > return x!=x ? 0 : 1;
> > }
> >
> > The C99 standard states in section 7.12.14:
> >
> > """
> > The relational and equality operators support the usual mathematical
> > relationships between numeric values. For any ordered pair of numeric
> > values exactly one of the relationships — less, greater, and equal — is
> > true. Relational operators may raise the ‘‘invalid’’ floating-point
> > exception when argument values are NaNs.
> > """
> >
> > My interpretation of that paragraph is that it's OK for <, <=, > and >= to
> > raise an exception when argument values are NaNs. It is not OK for == an !=
> > to raise an exception when argument values are NaNs.
> >
> > Therefore,
> >
> > int bar(float x) {
> > return x!=x ? 0 : 1;
> > }
> >
> > should not produce an exception when x is NaN, and hence a vcmp rather
> > than vcmpe instruction should be produced when generating ARM code for
> > this.
> >
> > http://llvm.org/viewvc/llvm-project?rev=294945&view=rev introduced
> > support for generating vcmp instead of vcmpe for equality comparisons.
> > How come vcmpe is generated for (x!=x)?
> >
> > The answer is that InstCombine transforms the equality comparison into an
> > "ordered comparison”. Before InstCombine:
> > define dso_local i32 @bar(float %x) local_unnamed_addr {
> > entry:
> > %cmp = fcmp une float %x, %x
> > %cond = select i1 %cmp, i32 0, i32 1
> > ret i32 %cond
> > }
> >
> > After InstCombine:
> > define dso_local i32 @bar(float %x) local_unnamed_addr #0 {
> > entry:
> > %cmp = fcmp ord float %x, 0.000000e+00
> > %cond = zext i1 %cmp to i32
> > ret i32 %cond
> > }
> >
> > Please note that on other backends like x86 or AArch64, this InstCombine
> > doesn’t trigger floating point exception behaviour since those backends
> > don’t seem to be producing any instructions for fcmp that raise floating
> > point exceptions on NaNs.
> >
> > My question here is: how to fix this behaviour? Or: which part in the
> > compilation flow is wrong?
> > Reading through various standards and specifications, I’m getting confused
> > to what the best fix would be:
> >
> >
> > - https://llvm.org/docs/LangRef.html#floating-point-environment states
> > "The default LLVM floating-point environment assumes that
> > floating-point instructions do not have side effects. Results assume the
> > round-to-nearest rounding mode. No floating-point exception state is
> > maintained in this environment. Therefore, there is no attempt to create or
> > preserve invalid operation (SNaN) or division-by-zero exceptions.”
> > This suggests that if we want to retain floating point exception
> > behaviour in the compilation flow, we shouldn’t be using the “default LLVM
> > floating-point environment”, but rather something else. Presumably the
> > constrained intrinsics? However, when I look at the constrained intrinsics
> > definition, it seems (
> > http://llvm.org/docs/LangRef.html#constrained-floating-point-intrinsics)
> > there is no constrained intrinsic for the floating point comparison
> > operation. Should there be one?
> > - If the default floating-point environment assumes that
> > floating-point instructions do not have side effects, why does the Arm
> > backend lower floating point comparison to vcmpe rather than vcmp? The
> > revision history suggests this has been this way since the initial creation
> > of the ARM backend. Should this behaviour be changed and vcmp be produced
> > rather than vcmpe? And only later, once the generation of constrained
> > floating point intrinsics is implemented should backends start producing
> > signalling floating point comparisons for floating point comparison
> > constrained intrinsics (assuming they’ll exist by then)?
> > - Or alternatively, there is a good reason to keep on producing vcmpe
> > as is today, and instcombine just shouldn’t convert “fcmp une” into “fcmp
> > ord”?
> > - Or as yet another alternative, instcombine is just fine converting
> > “fcmp une” into “fcmp ord”, and it’s the ARM backend that should produce
> > vcmp rather than vcmpe also for “unordered” comparisons, next to equality
> > comparisons?
> >
> >
> > Thanks,
> >
> > Kristof
> >
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