[PATCH] D62774: [DAGCombine][X86][AArch64][MIPS][LANAI] (C - x) - y -> C - (x + y) fold (PR41952)

[Roman Lebedev via Phabricator via llvm-commits]

lebedev.ri added inline comments.


================
Comment at: lib/CodeGen/SelectionDAG/DAGCombiner.cpp:3017
   }
+  // (C - x) - y  ->  C - (x + y)
+  if (N0.hasOneUse() && N0.getOpcode() == ISD::SUB &&
----------------
RKSimon wrote:
> Do we gain anything from the general case here? This seems to cause a small increase in register pressure in many cases - limit to the (0 - x) - y negation case only?
Could you please be more specific?
The only regressions i'm seeing are 32-bit `@reg64_lshr_by_sub_from_negated*`
The obvious positives are all these newly-formed `neg`
https://godbolt.org/z/JK0ldm <- which is kinda the endgoal here.

If we limit this to only hoist `neg` (and this will be the first such restriction
in all of these patches), then naturally, the `@reg64_lshr_by_sub_from_negated*`
pattern will no longer produce `neg`, since the `sub 64, %x` will not be sunk.

That can still be fixed, if we transform `C-X` to `neg(x)+C` or so in DAGCombine,
I was actually going to look into that, but it looked to not be necessary
with this patch in it's current form.


Repository:
  rL LLVM

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