[llvm] CodeGen: Add -denormal-fp-math-bf16 flag (PR #90425)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> Is this really a case of "defective instructions" or is it just a difference between the way that Intel processors understand the bfloat16 type compared to other architectures? 

As a format, it's just IEEE with a different combination of mantissa and exponent widths. Denormals have a specific, and clear meaning here and there's no implied flushing on computation 


> The Intel white paper on bfloat16 (https://www.intel.com/content/www/us/en/content-details/671279/bfloat16-hardware-numerics-definition.html) says, "There is no need to support denormals; FP32, and therefore also BF16, offer more than enough range for deep learning training tasks."

I don't know how to parse this comment. Denormals, in what type? I almost read this as "you don't need to handle fp16 denormals if you process in bf16 instead". At worst it's a subjective value judgement that bad behavior is OK, but I'm not sure that's what it's really saying 



https://github.com/llvm/llvm-project/pull/90425