RFR: 8308340: C2: Idealize Fma nodes

[Vladimir Kozlov]

On Wed, 21 Jun 2023 03:26:38 GMT, Fei Gao <fgao at openjdk.org> wrote:

> Some platforms, like aarch64, ppc, and riscv, support fusing `Math.fma(-a, b, c)` or `Math.fma(a, -b, c)` by generating partially symmetric match rules like:
> 
> 
>   match(Set dst (FmaF src3 (Binary (NegF src1) src2)));
>   match(Set dst (FmaF src3 (Binary src1 (NegF src2))));
> 
> 
> Since `Fma` is partially commutative, the patch is to convert `Math.fma(-a, b, c)` to `Math.fma(b, -a, c)` in gvn phase, making node patterns canonical. Then we can remove redundant rules.
> 
> Also, we should guarantee that C2 generates `Fma` nodes only on platforms supporting `Fma` instructions before matcher, so we can remove all `predicate(UseFMA)` for all `Fma` rules.
> 
> After the patch, the code size of libjvm.so on aarch64 platform decreased by 63.4k.
> 
> The patch passed all tier 1 - 3 on aarch64 and x86 platforms.

Check for `UseFMA` should be moved from `c2compiler.cpp` to `Matcher::match_rule_supported` in `.ad` files.
I see we have such check for Fma vectors in `x86.ad` but not for scalars.
Similar issue exist for other platforms.

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PR Review: https://git.openjdk.org/jdk/pull/14576#pullrequestreview-1491402018