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Merged
MaskRay merged 1 commit into from
Aug 8, 2025
Merged

[lld][ELF] filter out section symbols when use BP reorder #151685

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10 changes: 6 additions & 4 deletions lld/ELF/BPSectionOrderer.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -76,10 +76,12 @@ DenseMap<const InputSectionBase *, int> elf::runBalancedPartitioning(
if (!d)
return;
auto *sec = dyn_cast_or_null<InputSection>(d->section);
// Skip empty, discarded, ICF folded sections, .bss. Skipping ICF folded
// sections reduces duplicate detection work in BPSectionOrderer.
if (!sec || sec->size == 0 || !sec->isLive() || sec->repl != sec ||
!sec->content().data() || !orderer.secToSym.try_emplace(sec, d).second)
// Skip section symbols. Skip empty, discarded, ICF folded sections, .bss.
// Skipping ICF folded sections reduces duplicate detection work in
// BPSectionOrderer.
if (sym.isSection() || !sec || sec->size == 0 || !sec->isLive() ||
sec->repl != sec || !sec->content().data() ||
!orderer.secToSym.try_emplace(sec, d).second)
return;
rootSymbolToSectionIdxs[CachedHashStringRef(
lld::utils::getRootSymbol(sym.getName()))]
Expand Down
90 changes: 67 additions & 23 deletions lld/test/ELF/bp-section-orderer.s
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Original file line number Diff line number Diff line change
Expand Up @@ -21,38 +21,38 @@
# RUN: llvm-profdata merge a.proftext -o a.profdata
# RUN: ld.lld a.o --irpgo-profile=a.profdata --bp-startup-sort=function --verbose-bp-section-orderer --icf=all --gc-sections 2>&1 | FileCheck %s --check-prefix=STARTUP-FUNC-ORDER

# STARTUP-FUNC-ORDER: Ordered 3 sections ([[#]] bytes) using balanced partitioning
# STARTUP-FUNC-ORDER: Total area under the page fault curve: 3.
# STARTUP-FUNC-ORDER: Ordered 4 sections ([[#]] bytes) using balanced partitioning
# STARTUP-FUNC-ORDER: Total area under the page fault curve: 4.

# RUN: ld.lld -o out.s a.o --irpgo-profile=a.profdata --bp-startup-sort=function
# RUN: llvm-nm -jn out.s | tr '\n' , | FileCheck %s --check-prefix=STARTUP
# STARTUP: s5,s4,s3,s2,s1,A,B,C,F,E,D,merged1,merged2,_start,d4,d3,d2,d1,g1,{{$}}
# STARTUP: s5,s4,s3,s2,s1,A,B,C,L1,F,E,D,merged1,merged2,G,_start,d4,d3,d2,d1,g1,{{$}}

# RUN: ld.lld -o out.os a.o --irpgo-profile=a.profdata --bp-startup-sort=function --symbol-ordering-file a.txt
# RUN: llvm-nm -jn out.os | tr '\n' , | FileCheck %s --check-prefix=ORDER-STARTUP
# ORDER-STARTUP: s2,s1,s5,s4,s3,A,F,E,D,B,C,merged1,merged2,_start,d3,d2,d4,d1,g1,{{$}}
# ORDER-STARTUP: s2,s1,s5,s4,s3,A,F,E,D,B,C,L1,merged1,merged2,G,_start,d3,d2,d4,d1,g1,{{$}}

# RUN: ld.lld -o out.cf a.o --verbose-bp-section-orderer --bp-compression-sort=function 2>&1 | FileCheck %s --check-prefix=BP-COMPRESSION-FUNC
# RUN: ld.lld -o out.cf.icf a.o --verbose-bp-section-orderer --bp-compression-sort=function --icf=all --gc-sections 2>&1 | FileCheck %s --check-prefix=BP-COMPRESSION-ICF-FUNC
# RUN: llvm-nm -jn out.cf | tr '\n' , | FileCheck %s --check-prefix=CFUNC
# CFUNC: s5,s4,s3,s2,s1,A,F,merged1,merged2,C,E,D,B,_start,d4,d3,d2,d1,g1,{{$}}
# CFUNC: s5,s4,s3,s2,s1,C,E,D,B,G,F,merged1,merged2,A,_start,L1,d4,d3,d2,d1,g1,{{$}}

# RUN: ld.lld -o out.cd a.o --verbose-bp-section-orderer --bp-compression-sort=data 2>&1 | FileCheck %s --check-prefix=BP-COMPRESSION-DATA
# RUN: llvm-nm -jn out.cd | tr '\n' , | FileCheck %s --check-prefix=CDATA
# CDATA: s5,s3,s4,s2,s1,F,C,E,D,B,A,merged1,merged2,_start,d4,d1,d3,d2,g1,{{$}}
# CDATA: s5,s3,s4,s2,s1,F,C,E,D,B,A,merged1,merged2,L1,G,_start,d4,d1,d3,d2,g1,{{$}}

# RUN: ld.lld -o out.cb a.o --verbose-bp-section-orderer --bp-compression-sort=both 2>&1 | FileCheck %s --check-prefix=BP-COMPRESSION-BOTH
# RUN: llvm-nm -jn out.cb | tr '\n' , | FileCheck %s --check-prefix=CBOTH
# CBOTH: s5,s3,s4,s2,s1,A,F,merged1,merged2,C,E,D,B,_start,d4,d1,d3,d2,g1,{{$}}
# CBOTH: s5,s3,s4,s2,s1,C,E,D,B,G,F,merged1,merged2,A,_start,L1,d4,d1,d3,d2,g1,{{$}}

# RUN: ld.lld -o out.cbs a.o --verbose-bp-section-orderer --bp-compression-sort=both --irpgo-profile=a.profdata --bp-startup-sort=function 2>&1 | FileCheck %s --check-prefix=BP-COMPRESSION-BOTH
# RUN: llvm-nm -jn out.cbs | tr '\n' , | FileCheck %s --check-prefix=CBOTH-STARTUP
# CBOTH-STARTUP: s5,s3,s4,s2,s1,A,B,C,F,E,D,merged1,merged2,_start,d4,d1,d3,d2,g1,{{$}}
# CBOTH-STARTUP: s5,s3,s4,s2,s1,A,B,C,L1,F,E,D,merged1,merged2,G,_start,d4,d1,d3,d2,g1,{{$}}

# BP-COMPRESSION-FUNC: Ordered 9 sections ([[#]] bytes) using balanced partitioning
# BP-COMPRESSION-ICF-FUNC: Ordered 8 sections ([[#]] bytes) using balanced partitioning
# BP-COMPRESSION-FUNC: Ordered 11 sections ([[#]] bytes) using balanced partitioning
# BP-COMPRESSION-ICF-FUNC: Ordered 9 sections ([[#]] bytes) using balanced partitioning
# BP-COMPRESSION-DATA: Ordered 9 sections ([[#]] bytes) using balanced partitioning
# BP-COMPRESSION-BOTH: Ordered 18 sections ([[#]] bytes) using balanced partitioning
# BP-COMPRESSION-BOTH: Ordered 20 sections ([[#]] bytes) using balanced partitioning

#--- a.proftext
:ir
Expand All @@ -63,7 +63,7 @@
1
# Weight
1
A, B, C
A, B, C, L1

A
# Func Hash:
Expand Down Expand Up @@ -97,6 +97,14 @@ D
# Counter Values:
1

L1
# Func Hash:
5555
# Num Counters:
1
# Counter Values:
1

#--- a.txt
A
F
Expand Down Expand Up @@ -137,6 +145,9 @@ void A() {}
RETAIN int merged1(int a) { return F(a + 101); }
int merged2(int a) { return F(a + 101); }

RETAIN static int L1(int a) { return a + 103; }
int G(int a) { return L1(a); }

int _start() { return 0; }

#--- gen
Expand All @@ -148,7 +159,7 @@ clang --target=aarch64-linux-gnu -O0 -ffunction-sections -fdata-sections -fno-as
.p2align 2
.type F,@function
F: // @F
// %bb.0: // %entry
// %bb.0:
sub sp, sp, #32
stp x29, x30, [sp, #16] // 16-byte Folded Spill
add x29, sp, #16
Expand All @@ -167,7 +178,7 @@ F: // @F
.p2align 2
.type C,@function
C: // @C
// %bb.0: // %entry
// %bb.0:
sub sp, sp, #32
stp x29, x30, [sp, #16] // 16-byte Folded Spill
add x29, sp, #16
Expand All @@ -186,7 +197,7 @@ C: // @C
.p2align 2
.type E,@function
E: // @E
// %bb.0: // %entry
// %bb.0:
sub sp, sp, #32
stp x29, x30, [sp, #16] // 16-byte Folded Spill
add x29, sp, #16
Expand All @@ -205,7 +216,7 @@ E: // @E
.p2align 2
.type D,@function
D: // @D
// %bb.0: // %entry
// %bb.0:
sub sp, sp, #32
stp x29, x30, [sp, #16] // 16-byte Folded Spill
add x29, sp, #16
Expand All @@ -224,7 +235,7 @@ D: // @D
.p2align 2
.type B,@function
B: // @B
// %bb.0: // %entry
// %bb.0:
sub sp, sp, #32
stp x29, x30, [sp, #16] // 16-byte Folded Spill
add x29, sp, #16
Expand All @@ -243,7 +254,7 @@ B: // @B
.p2align 2
.type A,@function
A: // @A
// %bb.0: // %entry
// %bb.0:
ret
.Lfunc_end5:
.size A, .Lfunc_end5-A
Expand All @@ -253,7 +264,7 @@ A: // @A
.p2align 2
.type merged1,@function
merged1: // @merged1
// %bb.0: // %entry
// %bb.0:
sub sp, sp, #32
stp x29, x30, [sp, #16] // 16-byte Folded Spill
add x29, sp, #16
Expand All @@ -272,7 +283,7 @@ merged1: // @merged1
.p2align 2
.type merged2,@function
merged2: // @merged2
// %bb.0: // %entry
// %bb.0:
sub sp, sp, #32
stp x29, x30, [sp, #16] // 16-byte Folded Spill
add x29, sp, #16
Expand All @@ -286,16 +297,48 @@ merged2: // @merged2
.Lfunc_end7:
.size merged2, .Lfunc_end7-merged2
// -- End function
.section .text.L1,"axR",@progbits
.p2align 2 // -- Begin function L1
.type L1,@function
L1: // @L1
Comment on lines +300 to +303
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@ellishg ellishg Aug 1, 2025

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It doesn't look like this section has zero size? Can you also explain why these sections would have zero size?

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@Colibrow Colibrow Aug 1, 2025 •
edited
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Fix: Remove the d->size == 0 check and use sym.isSection() to skip reordering section symbols. I found a similar problem, see https://reviews.llvm.org/D59311?id=190644#inline-528590.

https://github.com/llvm/llvm-project/blob/f7226696027ec48266080549391982b1f9a840b6/lld/test/ELF/bp-section-orderer.s#L300-L313

This piece of code creates three symbols:

  • .text.L1: a STT_SECTION symbol where the symbol name is empty but d->section->size is not 0
  • L1: a STT_FUNC symbol, which is the one we want in Temporal Profiling
  • $x: a mapping symbol

These three symbols all match the section .text.L1, and sym.isSection() helps us filter out the unnamed section symbols in BP reorder.
@ellishg

// %bb.0:
sub sp, sp, #16
str w0, [sp, #12]
ldr w8, [sp, #12]
add w0, w8, #103
add sp, sp, #16
ret
.Lfunc_end8:
.size L1, .Lfunc_end8-L1
// -- End function
.section .text.G,"ax",@progbits
.globl G // -- Begin function G
.p2align 2
.type G,@function
G: // @G
// %bb.0:
sub sp, sp, #32
stp x29, x30, [sp, #16] // 16-byte Folded Spill
add x29, sp, #16
stur w0, [x29, #-4]
ldur w0, [x29, #-4]
bl L1
ldp x29, x30, [sp, #16] // 16-byte Folded Reload
add sp, sp, #32
ret
.Lfunc_end9:
.size G, .Lfunc_end9-G
// -- End function
.section .text._start,"ax",@progbits
.globl _start // -- Begin function _start
.p2align 2
.type _start,@function
_start: // @_start
// %bb.0: // %entry
// %bb.0:
mov w0, wzr
ret
.Lfunc_end8:
.size _start, .Lfunc_end8-_start
.Lfunc_end10:
.size _start, .Lfunc_end10-_start
// -- End function
.type s5,@object // @s5
.section .rodata.s5,"a",@progbits
Expand Down Expand Up @@ -395,3 +438,4 @@ g1:
.addrsig_sym B
.addrsig_sym A
.addrsig_sym merged1
.addrsig_sym L1