Skip to content

[MemProf] Don't mutate the function type when calling clone #147829

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Merged
teresajohnson merged 3 commits into from
Jul 11, 2025
Merged

[MemProf] Don't mutate the function type when calling clone #147829

Show file tree
Hide file tree
Changes from 2 commits
Commits
Show all changes
3 commits
Select commit Hold shift + click to select a range
ceca3b4
[MemProf] Don't mutate the function type when calling clone
teresajohnson Jul 9, 2025
b0f4b30
Address comments
teresajohnson Jul 11, 2025
706421c
Update statistic description
teresajohnson Jul 11, 2025
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
22 changes: 21 additions & 1 deletion llvm/lib/Transforms/IPO/MemProfContextDisambiguation.cpp
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -95,6 +95,7 @@ STATISTIC(NewMergedNodes, "Number of new nodes created during merging");
STATISTIC(NonNewMergedNodes, "Number of non new nodes used during merging");
STATISTIC(MissingAllocForContextId,
"Number of missing alloc nodes for context ids");
STATISTIC(SkippedCallsCloning, "Number of calls skipped during cloning");
Copy link

@snehasish snehasish Jul 11, 2025

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

I wonder if this could be more specific to the reason it was skipped? On the other hand, is this case the only place where we may skip calls during cloning?

Copy link
Contributor Author

@teresajohnson teresajohnson Jul 11, 2025

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

I started out making it longer and more specific, but figured we'd need to look up the reason in the code if we ever encounter this in the wild anyway (it's a developer-focused stat). Any suggestions for something more specific but not too terse? The other place we might skip is if it required ICP and we couldn't legally perform that. How about "Number of calls skipped during cloning due to unexpected operand" ?

Copy link

@snehasish snehasish Jul 11, 2025

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Maybe the ICP case is good to cover with this statistic too? Then we can keep the name and description as is.

Not a strong opinion, feel free to just update the description as you suggested as an alternative.

Copy link
Contributor Author

@teresajohnson teresajohnson Jul 11, 2025

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

I think I will update the description for this one as I suggested, since I'd like to know specifically if/when we hit this case. I'll add a separate stat for the other case(s) when I get a chance.


static cl::opt<std::string> DotFilePathPrefix(
"memprof-dot-file-path-prefix", cl::init(""), cl::Hidden,
Expand Down Expand Up @@ -5155,6 +5156,19 @@ bool MemProfContextDisambiguation::applyImport(Module &M) {

assert(!isMemProfClone(*CalledFunction));

// Because we update the cloned calls by calling setCalledOperand (see
// comment below), out of an abundance of caution make sure the called
// function was actually the called operand (or its aliasee). We also
// strip pointer casts when looking for calls (to match behavior during
// summary generation), however, with opaque pointers in theory this
// should not be an issue. Note we still clone the current function
// (containing this call) above, as that could be needed for its callers.
auto *GA = dyn_cast_or_null<GlobalAlias>(CB->getCalledOperand());
if (CalledFunction != CB->getCalledOperand() &&
(!GA || CalledFunction != GA->getAliaseeObject())) {
SkippedCallsCloning++;
return;
}
// Update the calls per the summary info.
// Save orig name since it gets updated in the first iteration
// below.
Expand All @@ -5173,7 +5187,13 @@ bool MemProfContextDisambiguation::applyImport(Module &M) {
CBClone = CB;
else
CBClone = cast<CallBase>((*VMaps[J - 1])[CB]);
CBClone->setCalledFunction(NewF);
// Set the called operand directly instead of calling setCalledFunction,
// as the latter mutates the function type on the call. In rare cases
// we may have a slightly different type on a callee function
// declaration due to it being imported from a different module with
// incomplete types. We really just want to change the name of the
// function to the clone, and not make any type changes.
CBClone->setCalledOperand(NewF.getCallee());
Copy link

@snehasish snehasish Jul 9, 2025

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Is it possible to assert that the type of the clone is the same as the original function here?

Copy link
Contributor Author

@teresajohnson teresajohnson Jul 11, 2025

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

I started to add such an assert, however, it seems unnecessary to me as we create NewF from the function type of CalledFunction just a few lines above here. However, looking at this made me concerned about the possibility of a different issue. Where we get CalledFunction we try various ways to find the function from the CallBase if getCalledFunction returns null. One of those things is to invoke stripPointerCasts. Now if we had a pointer cast here it would not be valid (pre-opaque pointers) to simply replace the called operand. But in theory, with opaque pointers, we should not actually find any pointer casts to strip. For testing, I added some checks in that code, as well as in the module summary analysis which does the same thing for all ThinLTO bitcode generation and kicks in much more broadly, and never found (with the regression tests or with a large app) any where it found anything to actually strip. But out of an abundance of caution I have added some handling above here to make sure the called function is the called operand (or its aliasee), and skip rewriting the call otherwise (also incrementing a new statistic in case we want to watch for cases this actually doing anything). In theory we could remove the stripPointerCasts call, but that needs to stay in sync with module summary analysis, so that's a change that could be considered later.

ORE.emit(OptimizationRemark(DEBUG_TYPE, "MemprofCall", CBClone)
<< ore::NV("Call", CBClone) << " in clone "
<< ore::NV("Caller", CBClone->getFunction())
Expand Down
62 changes: 62 additions & 0 deletions llvm/test/ThinLTO/X86/memprof_callee_type_mismatch.ll
View file
Open in desktop
Original file line number Diff line number Diff line change
@@ -0,0 +1,62 @@
;; Test to ensure the callite when updated to call a clone does not mutate the
;; callee function type. In rare cases we may end up with a callee declaration
;; that does not match the call type, because it was imported from a different
;; module with an incomplete return type (in which case clang gives it a void
;; return type).

; RUN: rm -rf %t && split-file %s %t && cd %t
; RUN: llvm-as src.ll -o src.o
; RUN: llvm-as src.o.thinlto.ll -o src.o.thinlto.bc
; RUN: opt -passes=memprof-context-disambiguation src.o -S -memprof-import-summary=src.o.thinlto.bc | FileCheck %s

;--- src.ll
; ModuleID = 'src.o'
source_filename = "src.c"
target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"

define i32 @main(ptr %b) {
entry:
;; This call is not changed as the summary specifies clone 0.
; CHECK: call ptr @_Z3foov()
%call = call ptr @_Z3foov(), !callsite !5
;; After changing this call to call a clone, the function type should still
;; be ptr, despite the void on the callee declaration.
; CHECK: call ptr @_Z3foov.memprof.1()
%call1 = call ptr @_Z3foov(), !callsite !6
%0 = load ptr, ptr %b, align 8
;; Although the summary indicates this should call clone 1, and the VP
;; metadata indicates the callee is _Z3foov, it is not updated because
;; the ICP facility requires the function types to match.
; CHECK: call ptr %0()
%call2 = call ptr %0(), !prof !7, !callsite !8
ret i32 0
}

;; Both the original callee function declaration and its clone have void return
;; type.
; CHECK: declare void @_Z3foov()
; CHECK: declare void @_Z3foov.memprof.1()
declare void @_Z3foov()

!llvm.dbg.cu = !{!0}
!llvm.module.flags = !{!2, !3, !4}

!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus_14, file: !1, producer: "clang version 21.0.0git ([email protected]:llvm/llvm-project.git e391301e0e4d9183fe06e69602e87b0bc889aeda)", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, splitDebugInlining: false, nameTableKind: None)
!1 = !DIFile(filename: "src.cc", directory: "", checksumkind: CSK_MD5, checksum: "8636c46e81402013b9d54e8307d2f149")
!2 = !{i32 7, !"Dwarf Version", i32 5}
!3 = !{i32 2, !"Debug Info Version", i32 3}
!4 = !{i32 1, !"EnableSplitLTOUnit", i32 0}
!5 = !{i64 8632435727821051414}
!6 = !{i64 -3421689549917153178}
!7 = !{!"VP", i32 0, i64 4, i64 9191153033785521275, i64 4}
!8 = !{i64 1234}

;--- src.o.thinlto.ll
; ModuleID = 'src.o.thinlto.bc'
source_filename = "src.o.thinlto.bc"

^0 = module: (path: "src.o", hash: (2823430083, 3994560862, 899296057, 1055405378, 2961356784))
^1 = gv: (guid: 15822663052811949562, summaries: (function: (module: ^0, flags: (linkage: external, visibility: default, notEligibleToImport: 0, live: 1, dsoLocal: 1, canAutoHide: 0, importType: definition), insts: 3, funcFlags: (readNone: 0, readOnly: 0, noRecurse: 0, returnDoesNotAlias: 0, noInline: 1, alwaysInline: 0, noUnwind: 0, mayThrow: 0, hasUnknownCall: 0, mustBeUnreachable: 0), callsites: ((callee: null, clones: (0), stackIds: (8632435727821051414)), (callee: null, clones: (1), stackIds: (15025054523792398438)), (callee: null, clones: (1), stackIds: (1234))))))
^2 = flags: 353
^3 = blockcount: 0
Loading