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:
> > 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.
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D80416/new/
https://reviews.llvm.org/D80416
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