`arith.truncf: f32 -> bf16` is lowered to "software" bf16 implementation about iree

[bjacob]

This rewrite is done by the MLIR Arith dialect pattern BFloat16TruncFOpConverter, added by arith::populateExpandBFloat16Patterns.

As of #15986 , we are only calling arith::populateExpandBFloat16Patterns once, in ConvertBf16ToUInt16BuffersPass.

If I drop ConvertBf16ToUInt16BuffersPass, as in this diff,

--- a/compiler/src/iree/compiler/Codegen/LLVMCPU/Passes.cpp
+++ b/compiler/src/iree/compiler/Codegen/LLVMCPU/Passes.cpp
@@ -592,7 +592,7 @@ static void addLowerToLLVMPasses(OpPassManager &passManager,
   }
   passManager.addNestedPass<func::FuncOp>(createConvertLinalgToLoopsPass());
   passManager.addPass(createConvertBf16ArithToF32Pass());
-  passManager.addPass(createConvertBf16ToUInt16BuffersPass());
+  //passManager.addPass(createConvertBf16ToUInt16BuffersPass());
   passManager.addNestedPass<func::FuncOp>(createCanonicalizerPass());
   passManager.addNestedPass<func::FuncOp>(createCSEPass());

then the testcase attached above in this PR compiles successfully and the assembly looks good:

	vcvtneps2bf16	(%rcx), %ymm0
	vmovaps	%ymm0, (%rax)
	vcvtneps2bf16	64(%rcx), %ymm0
	vmovaps	%ymm0, 32(%rax)

Omitting this pass also works beyond this toy testcase, e.g. it does allow me to run e2e LLama2 with --iree-global-opt-enable-demote-contraction-inputs-to-bf16 (for that I need some LLVM x86 fixes, llvm/llvm-project#76076). But the DT-but-not-UK e2e matmul tests with bf16 element type, run into more failures in the LLVM x86 backend.

So I would like to find a finer-granularity approach where I avoid the specific rewrite that I need to avoid (arith::populateExpandBFloat16Patterns) without omitting ConvertBf16ToUInt16BuffersPass altogether.

If I just comment out arith::populateExpandBFloat16Patterns in ConvertBf16ToUInt16BuffersPass, as in this diff,

--- a/compiler/src/iree/compiler/Codegen/Common/ConvertBf16ToUInt16Buffers.cpp
+++ b/compiler/src/iree/compiler/Codegen/Common/ConvertBf16ToUInt16Buffers.cpp
@@ -296,7 +296,7 @@ struct ConvertBf16ToUInt16BuffersPass final
       });
 
       RewritePatternSet patterns(ctx);
-      arith::populateExpandBFloat16Patterns(patterns);
+      //arith::populateExpandBFloat16Patterns(patterns);
       populateIreeBf16EmulationPatterns(patterns, typeConverter);
 
       if (failed(applyPartialConversion(op, target, std::move(patterns))))

then I get this error on the testcase attached in this PR:

/tmp/a.mlir:6:12: error: 'arith.truncf' op result #0 must be floating-point-like, but got 'vector<16xi16>'
      %5 = arith.truncf %in : f32 to bf16

It looks as if ConvertBf16ToUInt16Buffers just needs to insert some bitcast op to bridge between bf16 and i16?

I would be very glad for even a non-default pass option / iree-compile flag that I could use for Llama2 benchmarks locally.

@rsuderman