[all-commits] [llvm/llvm-project] 606d25: [FuncSpec] Compute specialisation gain even when f...

[Momchil Velikov via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 606d25e545d3d606371c47b20c233eef77bcc340
      https://github.com/llvm/llvm-project/commit/606d25e545d3d606371c47b20c233eef77bcc340
  Author: Momchil Velikov <momchil.velikov at arm.com>
  Date:   2022-10-26 (Wed, 26 Oct 2022)

  Changed paths:
    M llvm/lib/Transforms/IPO/FunctionSpecialization.cpp
    A llvm/test/Transforms/FunctionSpecialization/specialization-order.ll

  Log Message:
  -----------
  [FuncSpec] Compute specialisation gain even when forcing specialisation

When rewriting the call sites to call the new specialised functions, a
single call site can be matched by two different specialisations - a
"less specialised" version of the function and a "more specialised"
version of the function, e.g.  for a function

    void f(int x, int y)

the call like `f(1, 2)` could be matched by either

    void f.1(int x /* int y == 2 */);

or

    void f.2(/* int x == 1, int y == 2 */);

The `FunctionSpecialisation` pass tries to match specialisation in the
order of decreasing gain, so "more specialised" functions are
preferred to "less specialised" functions. This breaks, however, when
using the flag `-force-function-specialization`, in which case the
cost/benefit analysis is not performed and all the specialisations are
equally preferable.

This patch makes the pass calculate specialisation gain and order the
specialisations accordingly even when `-force-function-specialization`
is used, under the assumption that this flag has purely debugging
purpose and it is reasonable to ignore the extra computing effort it
incurs.

Reviewed By: ChuanqiXu, labrinea

Differential Revision: https://reviews.llvm.org/D136180