Cranelift: Optimize out redundant select + icmp instructions #12135
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This optimizes the following: a = select x, k1, k2 b = icmp eq a, k1 to b = x We shouldn't trigger this optimization when k1 == k2 because constant propagation should optimize that case.
Implement simple icmp + select optimization
* Optimize icmp eq and ne for select-on-icmp When we have a select followed by an icmp and the inner condition to the select is also an icmp, we can remove the select + icmp and directly use the inner condition. We negated the values of the inner_condition via bxor x 1. * Add trailing newline * feat: avoid relying on icmp inner_cond --------- Co-authored-by: school <[email protected]> Co-authored-by: Edward Wibowo <[email protected]>
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Collaborators: @claby2 @bisheshank
Cranelift may generate code where a
selectfollowed by anicmpcan be directly substituted with theselectinstruction's condition. Specifically, we can perform this optimization when we encounter code of the formHere, we can optimize to
v2 = v0given thatk1andk2are constants.In the language of the example above, this PR handles
icmp eqandicmp newhenk1ork2are theicmpcondition. We use the ISLE rewriteicmp ne x 0as a "cast" to handle the case where the inner condition of theselectis not restricted to 0 or 1 (Seenon_icmp_innerin issue-11578-semantics.clif)Fixes #11578