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Re: making the new if-converter not mangle IR that is already vectorizer-friendly


Abe wrote:
[snip]
Predication of instructions can help to remove the burden of the conditional branch, but is not available on all relevant architectures.
In some architectures that are typically implemented in modern high-speed processors -- i.e. with high core frequency, caches, speculative execution, etc. --
there is not full support for predication [as there is, for example, in 32-bit ARM] but there _is_ support for "conditional move" [hereinafter "cmove"].
If/when the ISA in question supports cmove to/from main memory, perhaps it would be profitable to use two cmoves back-to-back with opposite conditions
and the same register [destination for load, source for store] to implement e.g. "temp = c ? X[x] : Y[y]" and/or "temp = C[c] ? X[x] : Y[y]" etc.
Even without cmove to/from main memory, two cmoves back-to-back with opposite conditions could still be used, e.g. [not for a real-world ISA]:
   load X[x] -> reg1
   load Y[y] -> reg2
   cmove   c  ? reg1 -> reg3
   cmove (!c) ? reg2 -> reg3

Or even better if the ISA can support something like:
   load X[x] -> reg1
   load Y[y] -> reg2
   cmove (c ? reg1 : reg2) -> reg3

However, this is a code-gen issue from my POV, and at the present time all the if-conversion work is occurring at the GIMPLE level.
If anybody reading this knows how I could make the if converter generate GIMPLE that leads to code-gen that is better for at
least one ISA

Ramana writes that your patch for PR46029 generates more 'csel's (i.e. the second form you write) for AArch64: https://gcc.gnu.org/ml/gcc-patches/2015-06/msg01721.html

of course this says nothing about whether there is *some* other ISA that gets regressed!

HTH
Alan


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