[Metal] Make fast math mode optional (#3205)

* Add ability to toggle fast math mode in metal. Chose how to apply based on os version.

* Move available macro and friends to utils

* Isolate #[allow(deprecated)] to the actually deprecated method

* doc

* Use objc2::available macro instead

Author: ivarflakstad

candle-metal-kernels/src/kernel.rs

@@ -2,10 +2,13 @@ use crate::source::{
     AFFINE, BINARY, CAST, CONV, FILL, INDEXING, MLX_GEMM, MLX_SORT, QUANTIZED, RANDOM, REDUCE,
     SDPA, SORT, TERNARY, UNARY,
 };
+use crate::utils::get_env_bool;
 use crate::{
-    ComputePipeline, ConstantValues, Device, Function, Library, MTLCompileOptions, MTLMathMode,
-    MetalKernelError, Source,
+    ComputePipeline, ConstantValues, Device, Function, Library, MTLCompileOptions,
+    MTLMathFloatingPointFunctions, MTLMathMode, MetalKernelError, Source,
 };
+use objc2::available;
+use objc2::rc::Retained;
 use std::collections::HashMap;
 use std::sync::RwLock;
 
@@ -113,9 +116,7 @@ impl Kernels {
         } else {
             let lib = {
                 let source_content = self.get_library_source(source);
-                let compile_options = MTLCompileOptions::new();
-                //unsafe { compile_options.setEnableLogging(true) };
-                compile_options.setMathMode(MTLMathMode::Fast);
+                let compile_options = get_compile_options();
                 device
                     .new_library_with_source(source_content, Some(&compile_options))
                     .map_err(|e| MetalKernelError::LoadLibraryError(e.to_string()))?
@@ -176,3 +177,26 @@ impl Kernels {
         self.load_pipeline_with_constants(device, source, name, None)
     }
 }
+
+fn get_compile_options() -> Retained<MTLCompileOptions> {
+    let compile_options = MTLCompileOptions::new();
+    //unsafe { compile_options.setEnableLogging(true) };
+
+    let fast_math_enabled = get_env_bool("CANDLE_METAL_ENABLE_FAST_MATH", true);
+    // Ref availability:
+    // https://developer.apple.com/documentation/metal/mtlcompileoptions/mathmode
+    if available!(macos = 15, ios = 18) {
+        if fast_math_enabled {
+            compile_options.setMathMode(MTLMathMode::Fast);
+            compile_options.setMathFloatingPointFunctions(MTLMathFloatingPointFunctions::Fast);
+        } else {
+            compile_options.setMathMode(MTLMathMode::Relaxed);
+            compile_options.setMathFloatingPointFunctions(MTLMathFloatingPointFunctions::Precise);
+        }
+    } else {
+        // For older OS versions we use the old api
+        #[allow(deprecated)]
+        compile_options.setFastMathEnabled(fast_math_enabled);
+    }
+    compile_options
+}

candle-metal-kernels/src/lib.rs

@@ -16,7 +16,7 @@ use metal::{
     BlitCommandEncoder, Buffer, CommandQueue, ComputeCommandEncoder, ComputePipeline,
     ConstantValues, Device, Function, Library, MTLResourceOptions, Value,
 };
-use objc2_metal::{MTLCompileOptions, MTLMathMode, MTLSize};
+use objc2_metal::{MTLCompileOptions, MTLMathFloatingPointFunctions, MTLMathMode, MTLSize};
 use source::Source;
 pub use utils::BufferOffset;
 use utils::{get_block_dims, linear_split, EncoderParam, EncoderProvider};

candle-metal-kernels/src/utils.rs

@@ -1,5 +1,6 @@
 use crate::metal::{Buffer, CommandBuffer, ComputeCommandEncoder, ComputePipeline};
-use objc2_metal::MTLSize;
+use crate::MTLSize;
+use std::ffi::OsStr;
 use std::ops::Deref;
 use std::sync::{RwLockReadGuard, RwLockWriteGuard};
 
@@ -236,3 +237,14 @@ impl<'a, T> From<RwLockWriteGuard<'a, T>> for RwLockGuard<'a, T> {
         RwLockGuard::Write(g)
     }
 }
+
+fn is_truthy(s: String) -> bool {
+    match s.as_str() {
+        "true" | "t" | "yes" | "y" | "1" => true,
+        _ => false,
+    }
+}
+
+pub(crate) fn get_env_bool<K: AsRef<OsStr>>(key: K, default: bool) -> bool {
+    std::env::var(key).map(is_truthy).unwrap_or(default)
+}