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[WPD][ThinLTO]Add cutoff option for WPD #113383

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Oct 24, 2024
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[WPD][ThinLTO]Add cutoff option for WPD #113383

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[WPD][ThinLTO]Add cutoff option for WPD
mingmingl-llvm Oct 22, 2024
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mingmingl-llvm Oct 24, 2024
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12 changes: 12 additions & 0 deletions llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp
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Expand Up @@ -168,6 +168,14 @@ static cl::list<std::string>
cl::desc("Prevent function(s) from being devirtualized"),
cl::Hidden, cl::CommaSeparated);

/// If this value is other than 0, the devirt module pass will stop
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@teresajohnson teresajohnson Oct 23, 2024

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I believe the way the code is written, value 0 is not special and will cause no devirts to happen - assuming the option was specified (i.e. getNumOccurrences() > 0). And in fact we probably want a way to say "do 0 devirtualizations".

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@mingmingl-llvm mingmingl-llvm Oct 24, 2024

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I updated the comment, and yes it makes sense to do 0 devirtualization when this option is explicitly set to zero.

/// transformation once the total number of devirtualizations reach the cutoff
/// value.
static cl::opt<unsigned> WholeProgramDevirtCutoff(
"wholeprogramdevirt-cutoff",
cl::desc("Max number of devirtualization for devirt module pass"),
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@teresajohnson teresajohnson Oct 23, 2024

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s/devirtualization/devirtualizations/

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@mingmingl-llvm mingmingl-llvm Oct 24, 2024

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done.

cl::init(0));

/// Mechanism to add runtime checking of devirtualization decisions, optionally
/// trapping or falling back to indirect call on any that are not correct.
/// Trapping mode is useful for debugging undefined behavior leading to failures
Expand Down Expand Up @@ -1169,6 +1177,10 @@ void DevirtModule::applySingleImplDevirt(VTableSlotInfo &SlotInfo,
if (!OptimizedCalls.insert(&VCallSite.CB).second)
continue;

if (WholeProgramDevirtCutoff.getNumOccurrences() > 0 &&
NumSingleImpl >= WholeProgramDevirtCutoff)
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@teresajohnson teresajohnson Oct 23, 2024

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NumSingleImpl is a STATISTIC - I'm not sure if it is safe to use this here as I believe the compiler can be built with statistics disabled, in which case this variable would not be correctly tracking the number.

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@mingmingl-llvm mingmingl-llvm Oct 24, 2024

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Thanks for the catch!

I added static unsigned NumDevirtCalls to track the number of de-virtualized calls.

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@mingmingl-llvm mingmingl-llvm Oct 24, 2024

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I tested the updated PR (using static unsigned NumDevirtCalls) using clang and verified the new option works

  • -wholeprogramdevirt-cutoff=0 gives no devirtualization
  • -wholeprogramdevirt-cutoff=1 allows at most one in one clang invocation (and catches the potentially interesting call-site)

return;

if (RemarksEnabled)
VCallSite.emitRemark("single-impl",
TheFn->stripPointerCasts()->getName(), OREGetter);
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6 changes: 6 additions & 0 deletions llvm/test/Transforms/WholeProgramDevirt/import.ll
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Expand Up @@ -8,6 +8,12 @@
; RUN: opt -S -passes=wholeprogramdevirt -wholeprogramdevirt-summary-action=import -wholeprogramdevirt-read-summary=%S/Inputs/import-vcp-branch-funnel.yaml < %s | FileCheck --check-prefixes=CHECK,VCP,VCP-X86,VCP64,BRANCH-FUNNEL %s
; RUN: opt -S -passes=wholeprogramdevirt -wholeprogramdevirt-summary-action=import -wholeprogramdevirt-read-summary=%S/Inputs/import-branch-funnel.yaml < %s | FileCheck --check-prefixes=CHECK,BRANCH-FUNNEL,BRANCH-FUNNEL-NOVCP %s

; Cutoff value is not explicitly set. Expect 3 remark messages.
; RUN: opt -S -passes=wholeprogramdevirt -wholeprogramdevirt-summary-action=import -pass-remarks=wholeprogramdevirt -wholeprogramdevirt-read-summary=%S/Inputs/import-single-impl.yaml < %s 2>&1 | grep "single-impl" | count 3
; Cutoff value is set to 1. Expect one remark messages.
; RUN: opt -S -passes=wholeprogramdevirt -wholeprogramdevirt-summary-action=import -pass-remarks=wholeprogramdevirt -wholeprogramdevirt-cutoff=1 -wholeprogramdevirt-read-summary=%S/Inputs/import-single-impl.yaml < %s 2>&1 | grep "single-impl" | count 1
; Cutoff value is explicitly set to zero. Expect no remark message.
; RUN: opt -S -passes=wholeprogramdevirt -wholeprogramdevirt-summary-action=import -pass-remarks=wholeprogramdevirt -wholeprogramdevirt-cutoff=0 -wholeprogramdevirt-read-summary=%S/Inputs/import-single-impl.yaml < %s 2>&1 | FileCheck -implicit-check-not="remark" %s
target datalayout = "e-p:64:64"
target triple = "x86_64-unknown-linux-gnu"

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