[llvm-dev] RFC: Inlining report

[Cox, Robert via llvm-dev]

RFC: Inlining Report

Motivation

Making good inlining choices while optimizing an application is often key to achieving optimal performance.  While the compiler's default inlining heuristics sometimes provide great out-of-box results, optimal performance is sometimes achieved only after varying the settings of certain compiler options related to inlining or adding "always_inline" or "noinline" attributes to certain functions.

Before we can determine how we need change the compiler's inlining choices to get better performance for an application, we need to have a clear picture of the compiler's inlining choices and what motivated them.  Many compilers like LLVM and GCC provide informational notes when a function is inlined, but  these notes provide only a "blow by blow" description of what the compiler did, rather than a high level illustration of the result. This high level picture can be provided by an inlining report.

Over the years, I've worked with several compilers that provide inlining reports, and I can attest that the customers using those compilers have found them to be invaluable tool in investigating and improving their applications' performance.  In addition,  the inlining report can be used by compiler developers to visualize and improve the compiler's default heuristics and option values.

For these reasons, I'd like to contribute code to LLVM to generate an inlining report as part of the inliner.

Description

The inlining report I am proposing contains the following information:


(1)    The values of the principle threshold options which affect how much inlining is done under various circumstances

(2)    Whether each function is compiled or has been eliminated by dead static function elimination.

(3)    For each function, the call sites that were and were not inlined.  Since inlining a call site can expose other call sites for inlining, the inlining report also reports on whether these exposed call sites have been inlined or not.  This information is presented in hierarchical manner.

(4)    For each call site, we include the principle reason the call site was or was not inlined, together with any cost vs . threshold computation that was done.

High Level Design

The inline report is created if the option -inline-report=X is passed on command line with a positive integer value of X.  If X is 0, or this option is not specified, the Inliner does not create or perform any operations on the inline report, and there is no compile time overhead.

Three main classes are used to implement the inline report:

class InlineReportCallSite

This class contains the inlining report information specific to a particular CallSite CS, including:

(1)    A bool indicating whether or not the CallSite was or was not inlined

(2)    An inlining reason  indicating why the CallSite was or was not inlined

(3)    The inlining cost, outer inlining cost, and threshold values used in calculating the profitability of inlining

(4)    A vector of InlineReportCallSite*, each of which points to an InlineReportCallSite for a CallSite exposed when CS was inlined.

class InlineReportFunction

This class contains the inlining report information specific to a particular Function F in the call graph, including:

(1)    A bool indicating whether the function has been dead static eliminated.

(2)    A vector of call InlineReportCallSite*, each of which points to an InlineReportCallSite for a CallSite that appeared in F before any inlining was applied.

class InlineReport

The main class which summarizes the high level information in the inline report, including:

(1)    The values of the inlining threshold options

(2)    The "level" of the inlining report, which is a bit vector of feature options.  For example, whether to print external functions and intrinsics, whether to print the inlining reasons, etc.

(3)    A map MF from each Function* to InlineReportFunction*

(4)    A map MCS from each CallSite* to InlineReportCallSite*

In addition, the class InlineCost (from InlineCost.h) is augmented to include the primary reason a call site was inlined.

The class Inliner has been augmented with an InlineReport, which is created when an Inliner is constructed. The InlineReport is updated using calls to the member functions of these three classes in Inliner::runOnSCC() and the functions called by it.

Before any inlining is done in a particular call to runOnSCC(), the map MF is updated so that each Function (caller or callee) that will be examined for inlining has a corresponding InlineReportFunction in the map.  (The map MCS is also updated in a similar way, but only when a Function is actually inlined.)

The Inliner determines if a CallSite should be inlined by first calling Inliner::ShouldInline().   This calls getInlineCost() which returns an InlineCost, which now includes the reason the call site should or should not be inlined.  This reason, as well and costs and threshold from the InlineCost are stored in the InlineReportCallSite for the CallSite.

Then Inliner calls the static function InlineCallPossible(). If the inlining was not performed, the reason for not inlining is recorded in the InlineReportCallSite corresponding to the CallSite.  If the inlining was performed, the corresponding InlineReportCallSite is marked as inlined, and it is populated with the InlineReportCallSites corresponding to the newly exposed CallSites that were created during the inlining.

The InlineReport is printed during the call to Inliner::doFinalization().

Since the compiler can run any number of optimizations between two successive calls to runOnSCC(), the Instructions corresponding to CallSites can be deleted by the optimizations.  Callbacks are used to mark the corresponding InlineReportCallSites as deleted when this happens.

Example

Here is an example of abbreviated inlining report that is generated in my locally modified copy of the LLVM sources.  I generated this by compiling the file bzip2.c from the spec 2006 benchmark 401.bzip.  (For the sake of brevity, I didn't include all of the report.  Omitted parts are indicated by .... in the report.)

---- Begin Inlining Report ----

Option Values:
  inline-threshold: 225
  inlinehint-threshold: 325
  inlinecold-threshold: 225
  inlineoptsize-threshold: 15

COMPILE FUNC: fopen_output_safely
   -> EXTERN: open
   -> EXTERN: fdopen
   -> EXTERN: close

DEAD STATIC FUNC: setExit

DEAD STATIC FUNC: copyFileName

DEAD STATIC FUNC: showFileNames

DEAD STATIC FUNC: stat

....

COMPILE FUNC: cleanUpAndFail
   -> llvm.lifetime.start
      [[Callee is intrinsic]]
   -> INLINE: stat (35<=487)
      <<Callee is single basic block>>
      -> EXTERN: __xstat
   -> EXTERN: fprintf
   -> EXTERN: fclose
   -> EXTERN: remove
   -> EXTERN: fprintf
   -> EXTERN: fprintf
   -> EXTERN: fprintf
   -> EXTERN: fprintf
   -> EXTERN: fprintf
   -> EXTERN: fprintf
   -> INLINE: setExit (15<=225)
      <<Inlining is profitable>>
   -> EXTERN: exit

....

COMPILE FUNC: outOfMemory
   -> EXTERN: fprintf
   -> INLINE: showFileNames (70<=225)
      <<Inlining is profitable>>
      -> EXTERN: fprintf
   -> cleanUpAndFail
      [[Callee is noreturn]]

....

COMPILE FUNC: snocString
   -> INLINE: mkCell (-14920<=225)
      <<Callee has single callsite and local linkage>>
      -> INLINE: myMalloc (70<=225)
         <<Inlining is profitable>>
         -> EXTERN: malloc
         -> outOfMemory
            [[Callee is noreturn]]
   -> EXTERN: strlen
   -> INLINE: myMalloc (-14925<=225)
      <<Callee has single callsite and local linkage>>
      -> EXTERN: malloc
      -> outOfMemory
         [[Callee is noreturn]]
   -> EXTERN: strcpy
   -> snocString
      [[Callee is never inline]]

.....

---- End Inlining Report ------

Here is an explanation of some of the features:


(1)    Option values

Option Values:
  inline-threshold: 225
  inlinehint-threshold: 325
  inlinecold-threshold: 225
  inlineoptsize-threshold: 15

The report begins with a list of the most relevant option values to inlining.


(2)    Compiled and dead functions

COMPILE FUNC: fopen_output_safely
   -> EXTERN: open
   -> EXTERN: fdopen
   -> EXTERN: close

DEAD STATIC FUNC: setExit

Functions in the file are identified as either being compiled or eliminated by dead static function elimination.


(3)    External function calls

COMPILE FUNC: fopen_output_safely
   -> EXTERN: open
   -> EXTERN: fdopen
   -> EXTERN: close

Calls to externally defined functions are indicated by the word EXTERN.  These lines can optionally be omitted.


(4)    Inlining and nesting

COMPILE FUNC: snocString
   -> INLINE: mkCell (-14920<=225)
      <<Callee has single callsite and local linkage>>
      -> INLINE: myMalloc (70<=225)
         <<Inlining is profitable>>
         -> EXTERN: malloc

Inlined functions are marked INLINE. The inlining of a function within other inlined functions is shown clearly in the report using indentation.


(5)    Reasons functions were and were not inlined

COMPILE FUNC: cleanUpAndFail
   -> llvm.lifetime.start
      [[Callee is intrinsic]]
   -> INLINE: stat (35<=487)
      <<Callee is single basic block>>
      -> EXTERN: __xstat
   -> EXTERN: fprintf
   -> EXTERN: fclose
   -> EXTERN: remove
   -> EXTERN: fprintf
   -> EXTERN: fprintf
   -> EXTERN: fprintf
   -> EXTERN: fprintf
   -> EXTERN: fprintf
   -> EXTERN: fprintf
   -> INLINE: setExit (15<=225)
      <<Inlining is profitable>>
   -> EXTERN: exit

....

COMPILE FUNC: outOfMemory
   -> EXTERN: fprintf
   -> INLINE: showFileNames (70<=225)
      <<Inlining is profitable>>
      -> EXTERN: fprintf
   -> cleanUpAndFail
      [[Callee is noreturn]]

The principal reason a function was or was not inlined can be optionally displayed in the report.  The reason a function was inlined is indicated in double angle brackets << >>.  The reason a function was not inlined is indicated in double square brackets [[ ]].   When a comparison of the cost and threshold was used to determine if the function should be inlined, the comparison done is given.   (Since intrinsics are never inlined, information about them can be suppressed in the report.) The reasons for or for not inlining can optionally be displayed on the same line as the function considered for inlining for easy analysis using grep, awk, etc.


(6)    Line and column info

COMPILE FUNC: outOfMemory
   -> EXTERN: fprintf   bzip2.c(1016,4)
   -> showFileNames   bzip2.c(1019,4) [[Callee is never inline]]
   -> cleanUpAndFail   bzip2.c(1020,4) [[Callee is never inline]]

Optionally, file, line, and column info can be provided for call sites if source position information is present (using -g or
-gline-tables-only).

I would appreciate any comments you have on whether you support the inclusion of an inline report in LLVM, the form and features I have outlined above, and your thoughts on the high level design.

Thank you in advance for your comments,

Robert Cox
robert.cox at intel.com<mailto:robert.cox at intel.com>


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