Anastasia marked an inline comment as done.
Anastasia added inline comments.
================
Comment at: clang/test/CodeGenOpenCL/relaxed-fpmath.cl:21
+float fused_mad(float a, float b, float c) {
+ // NORMAL: @llvm.fmuladd.f32
+ // FAST: fmul fast float
----------------
Anastasia wrote:
> Anastasia wrote:
> > Anastasia wrote:
> > > I don't find this behavior "NORMAL". I don't believe we should contract
> > > expressions by default in OpenCL...
> > I just found in table 38 of OpenCL C spec, last entry says:
> >
> >
> > > x * y + z
> > > Implemented either as a correctly rounded fma or as a multiply and an add
> > > both of which are correctly rounded.
> >
> > In table 8 for `fma` it states:
> >
> > > Returns the correctly rounded floating-point representation of the sum of
> > > c with the infinitely precise product of a and b. Rounding of
> > > intermediate products shall not occur.
> >
> >
> > When I check LLVM doecumentation for fmuladd it says:
> >
> > > is equivalent to the expression a * b + c, except that it is unspecified
> > > whether rounding will be performed between the multiplication and
> > > addition steps. Fusion is not guaranteed, even if the target platform
> > > supports it. If a fused multiply-add is required, the corresponding
> > > llvm.fma intrinsic function should be used instead. This never sets
> > > errno, just as ‘llvm.fma.*’.
> >
> > Does this mean that rounding of an intermediate product may occur and
> > therefore it is not safe to use it for OpenCL mode by default?
> >
> After more digging I conclude that the use of `fmuladd` for `x * y + z`
> should be ok as per table 38 of OpenCL C because it allows either fused or
> non-fused operation i.e. it is ok if either intermediate value is rounded or
> not.
I changed my mind again. This is now covered by https://reviews.llvm.org/D80440.
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D80416/new/
https://reviews.llvm.org/D80416
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