The conditions for which Clang emits the `unsafe-fp-math` function
attribute has been modified as part of
`84a9ec2ff1ee97fd7e8ed988f5e7b197aab84a7`.
In the backend code generators `"unsafe-fp-math"="true"` enable floating
point contraction for the whole function.
The intent of the change in `84a9ec2ff1ee97fd7e8ed988f5e7b197aab84a7`
was to prevent backend code generators performing contractions when that
is not expected.
However the change is inaccurate and incomplete because it allows
`unsafe-fp-math` to be set also when only in-statement contraction is
allowed.
Consider the following example
```
float foo(float a, float b, float c) {
float tmp = a * b;
return tmp + c;
}
```
and compile it with the command line
```
clang -fno-math-errno -funsafe-math-optimizations -ffp-contract=on \
-O2 -mavx512f -S -o -
```
The resulting assembly has a `vfmadd213ss` instruction which corresponds
to a fused multiply-add. From the user perspective there shouldn't be
any contraction because the multiplication and the addition are not in
the same statement.
The optimized IR is:
```
define float @test(float noundef %a, float noundef %b, float noundef %c) #0 {
%mul = fmul reassoc nsz arcp afn float %b, %a
%add = fadd reassoc nsz arcp afn float %mul, %c
ret float %add
}
attributes #0 = {
[...]
"no-signed-zeros-fp-math"="true"
"no-trapping-math"="true"
[...]
"unsafe-fp-math"="true"
}
```
The `"unsafe-fp-math"="true"` function attribute allows the backend code
generator to perform `(fadd (fmul a, b), c) -> (fmadd a, b, c)`.
In the current IR representation there is no way to determine the
statement boundaries from the original source code.
Because of this for in-statement only contraction the generated IR
doesn't have instructions with the `contract` fast-math flag and
`llvm.fmuladd` is being used to represent contractions opportunities
that occur within a single statement.
Therefore `"unsafe-fp-math"="true"` can only be emitted when contraction
across statements is allowed.
Moreover the change in `84a9ec2ff1ee97fd7e8ed988f5e7b197aab84a7` doesn't
take into account that the floating point math function attributes can
be refined during IR code generation of a function to handle the cases
where the floating point math options are modified within a compound
statement via pragmas (see `CGFPOptionsRAII`).
For consistency `unsafe-fp-math` needs to be disabled if the contraction
mode for any scope/operation is not `fast`.
Similarly for consistency reason the initialization of `UnsafeFPMath` of
in `TargetOptions` for the backend code generation should take into
account the contraction mode as well.
Reviewed By: zahiraam
Differential Revision: https://reviews.llvm.org/D136786
There are currently two options that are used to tell the compiler to perform
unsafe floating-point optimizations:
'-ffast-math' and '-funsafe-math-optimizations'.
'-ffast-math' is enabled by default. It automatically enables the driver option
'-menable-unsafe-fp-math'.
Below is a table illustrating the special operations enabled automatically by
'-ffast-math', '-funsafe-math-optimizations' and '-menable-unsafe-fp-math'
respectively.
Special Operations -ffast-math -funsafe-math-optimizations -menable-unsafe-fp-math
MathErrno 0 1 1
FiniteMathOnly 1 0 0
AllowFPReassoc 1 1 1
NoSignedZero 1 1 1
AllowRecip 1 1 1
ApproxFunc 1 1 1
RoundingMath 0 0 0
UnsafeFPMath 1 0 1
FPContract fast on on
'-ffast-math' enables '-fno-math-errno', '-ffinite-math-only',
'-funsafe-math-optimzations' and sets 'FpContract' to 'fast'. The driver option
'-menable-unsafe-fp-math' enables the same special options than
'-funsafe-math-optimizations'. This is redundant.
We propose to remove the driver option '-menable-unsafe-fp-math' and use
instead, the setting of the special operations to set the function attribute
'unsafe-fp-math'. This attribute will be enabled only if those special
operations are enabled and if 'FPContract' is either 'fast' or set to the
default value.
Differential Revision: https://reviews.llvm.org/D135097
Michele Scandale