[PATCH] D24414: Conditionally eliminate library calls where the result value is not used

[Rong Xu via llvm-commits]

xur created this revision.
xur added reviewers: davidxl, hfinkel, mehdi_amini.
xur added subscribers: llvm-commits, xur.
Herald added subscribers: beanz, mehdi_amini.

This pass shrink-wraps a condition to some library calls where the call
result is not used. For example:
   sqrt(val);
 is transformed to
   if (val < 0)
     sqrt(val);
Even if the result of library call is not being used, the compiler cannot
safely delete the call because the function can set errno on error
conditions.
Note in many functions, the error condition solely depends on the incoming
parameter. In this optimization, we can generate the condition can lead to
the errno to shrink-wrap the call. Since the chances of hitting the error
condition is low, the runtime call is effectively eliminated.

These partially dead calls are usually results of C++ abstraction penalty
exposed by inlining. This optimization hits 108 times in 19 C/C++ programs
in SPEC2006.

https://reviews.llvm.org/D24414

Files:
  include/llvm/InitializePasses.h
  include/llvm/LinkAllPasses.h
  include/llvm/Transforms/Scalar.h
  include/llvm/Transforms/Utils/CondDeadCallElimination.h
  lib/Passes/PassBuilder.cpp
  lib/Passes/PassRegistry.def
  lib/Transforms/IPO/PassManagerBuilder.cpp
  lib/Transforms/Utils/CMakeLists.txt
  lib/Transforms/Utils/CondDeadCallElimination.cpp
  lib/Transforms/Utils/Utils.cpp
  test/Transforms/Util/cond-call-dce-double.ll
  test/Transforms/Util/cond-call-dce-float.ll
  test/Transforms/Util/cond-call-dce-long-double.ll

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