[llvm-bugs] [Bug 36404] New: Conditionalize code gen based on hotness/coldness

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https://bugs.llvm.org/show_bug.cgi?id=36404

            Bug ID: 36404
           Summary: Conditionalize code gen based on hotness/coldness
           Product: libraries
           Version: trunk
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: GlobalISel
          Assignee: unassignedbugs at nondot.org
          Reporter: dave at znu.io
                CC: llvm-bugs at lists.llvm.org

This bug is a followup to discussion in:
  https://reviews.llvm.org/D43201

It would be great if the compiler could conditionalize code gen based on the
hot/cold nature of functions and/or basic blocks. Anecdotally speaking (from my
experience at Apple), improving overall code density tends to improve overall
system performance. That being said, microbenchmarks sometimes suffer.

It would be great if LLVM could optimize for "reasonable size" (i.e. not
micro-coded; a.k.a. -Os not -Oz) by default and then switch to throughput
optimized code gen when evidence suggests that a "hot path" exists. At best,
evidence can include profile guided optimization and explicit source
attributes; and at worst simple heuristics.

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