[Mlir-commits] [mlir] [MLIR][Math] add canonicalize-f32-promotion pass (PR #92482)

[Mehdi Amini]

================
@@ -36,4 +36,51 @@ def MathLegalizeToF32 : Pass<"math-legalize-to-f32"> {
   let dependentDialects = ["math::MathDialect", "arith::ArithDialect"];
 }
 
+def MathCanonicalizeF32Promotion : Pass<"math-canonicalize-f32-promotion"> {
+  let summary = "Eliminate redundant truncf/extf pairs";
+  let description = [{
+    `legalize-to-f32` pass does f32 promotion for every op belonging to the
+    illegal op list. Once there are some consecutive illegal ops, `legalize-to-f32`
+    will insert redundant `arith.truncf` and `arith.extf` pairs between the illegal
+    ops.
+    
+    This pass is to eliminate the redundant truncf/extf pairs to improve
+    performance.
+
+    However, this pass may introduce numerical difference as the `f32->bf16` rounding
----------------
joker-eph wrote:

> can't be simplified to return %arg0, even under fastmath, because that is an explicit, user-specified desire to lose precision.

Sorry but I don't quite see why fast-math does not allow this in LLVM IR?
I don't really buy the "user-specified desired" because patterns of IR are happening in LLVM after quite a churn of optimization (possibly only through inlining, etc.). 
If the user *really* want something, they shouldn't decorate these with fast math!

https://github.com/llvm/llvm-project/pull/92482