Merge pull request #38034 from JuliaLang/tb/fast_half

Let Float16 participate in at-fastmath.

base/fastmath.jl

@@ -158,7 +158,7 @@ end
 
 # Basic arithmetic
 
-const FloatTypes = Union{Float32,Float64}
+const FloatTypes = Union{Float16,Float32,Float64}
 
 sub_fast(x::FloatTypes) = neg_float_fast(x)
 

src/llvm-demote-float16.cpp

@@ -57,6 +57,11 @@ bool DemoteFloat16Pass::runOnFunction(Function &F)
                 continue;
             }
 
+            // skip @fastmath operations
+            // TODO: more fine-grained check (afn?)
+            if (I.isFast())
+                continue;
+
             IRBuilder<> builder(&I);
 
             // extend Float16 operands to Float32

test/llvmpasses/fastmath.jl

@@ -16,3 +16,19 @@ import Base.FastMath
 
 # CHECK: call fast float @llvm.sqrt.f32(float %0)
 emit(FastMath.sqrt_fast, Float32)
+
+
+# Float16 operations should be performed as Float32, unless @fastmath is specified
+# TODO: this is not true for platforms that natively support Float16
+
+foo(x::T,y::T) where T = x-y == zero(T)
+# LOWER: fsub half %0, %1
+# FINAL: %2 = fpext half %0 to float
+# FINAL: %3 = fpext half %1 to float
+# FINAL: fsub half %2, %3
+emit(foo, Float16, Float16)
+
+@fastmath foo(x::T,y::T) where T = x-y == zero(T)
+# LOWER: fsub fast half %0, %1
+# FINAL: fsub fast half %0, %1
+emit(foo, Float16, Float16)