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[lld-macho] Choose ICF root function deterministically based on symbol names #158157

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d02f446
[lld-macho] Choose ICF root function deterministically based on symbo…
kyulee-com Sep 11, 2025
1768689
Simplify the logic
kyulee-com Sep 12, 2025
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30 changes: 27 additions & 3 deletions lld/MachO/ICF.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -408,11 +408,35 @@ void ICF::run() {
// When using safe_thunks, ensure that we first sort by icfEqClass and
// then by keepUnique (descending). This guarantees that within an
// equivalence class, the keepUnique inputs are always first.
if (config->icfLevel == ICFLevel::safe_thunks)
if (a->icfEqClass[0] == b->icfEqClass[0])
return a->keepUnique > b->keepUnique;
if (a->icfEqClass[0] == b->icfEqClass[0]) {
if (config->icfLevel == ICFLevel::safe_thunks) {
if (a->keepUnique != b->keepUnique)
return a->keepUnique > b->keepUnique;
}
// Within each equivalence class, sort by symbol name for determinism.
// If a section has an external symbol at offset 0, use that name.
// If either section doesn't have such a symbol, retain the original
// order.
auto getExternalSymbolName =
[](const ConcatInputSection *isec) -> std::optional<StringRef> {
for (const Symbol *sym : isec->symbols) {
if (const auto *d = dyn_cast<Defined>(sym)) {
if (d->value == 0 && d->isExternal())
return d->getName();
}
}
return std::nullopt;
};
auto nameA = getExternalSymbolName(a);
auto nameB = getExternalSymbolName(b);
// For determinism, define a strict ordering only when both sections
// have a primary symbol. Otherwise, they are considered equivalent
// (the expression returns false), preserving the stable sort order.
return nameA && nameB && *nameA < *nameB;
}
return a->icfEqClass[0] < b->icfEqClass[0];
});

forEachClass([&](size_t begin, size_t end) {
segregate(begin, end, &ICF::equalsConstant);
});
Expand Down
22 changes: 11 additions & 11 deletions lld/test/MachO/arm64-thunks.s
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Original file line number Diff line number Diff line change
Expand Up @@ -40,7 +40,7 @@
# OBJC: _objc_msgSend$bar:
# OBJC: _objc_msgSend$foo:

# MAP: 0x{{[[:xdigit:]]+}} {{.*}} _fold_func_low_addr
# MAP: 0x{{[[:xdigit:]]+}} {{.*}} _fold_func_first_addr
# MAP-NEXT: 0x{{[[:xdigit:]]+}} {{.*}} _a
# MAP-NEXT: 0x{{[[:xdigit:]]+}} {{.*}} _b
# MAP-NEXT: 0x{{[[:xdigit:]]+}} {{.*}} _c
Expand All @@ -58,12 +58,12 @@
# MAP-NEXT: 0x{{[[:xdigit:]]+}} {{.*}} _b.thunk.0
# MAP-NEXT: 0x{{[[:xdigit:]]+}} {{.*}} _h
# MAP-NEXT: 0x{{[[:xdigit:]]+}} {{.*}} _main
# MAP-NEXT: 0x{{[[:xdigit:]]+}} {{.*}} _fold_func_high_addr
# MAP-NEXT: 0x{{[[:xdigit:]]+}} {{.*}} _fold_func_second_addr
# MAP-NEXT: 0x{{[[:xdigit:]]+}} {{.*}} _c.thunk.0
# MAP-NEXT: 0x{{[[:xdigit:]]+}} {{.*}} _d.thunk.1
# MAP-NEXT: 0x{{[[:xdigit:]]+}} {{.*}} _e.thunk.1
# MAP-NEXT: 0x{{[[:xdigit:]]+}} {{.*}} _f.thunk.1
# MAP-NEXT: 0x{{[[:xdigit:]]+}} {{.*}} _fold_func_low_addr.thunk.0
# MAP-NEXT: 0x{{[[:xdigit:]]+}} {{.*}} _fold_func_first_addr.thunk.0
# MAP-NEXT: 0x{{[[:xdigit:]]+}} {{.*}} _z


Expand Down Expand Up @@ -243,14 +243,14 @@ _objc_msgSend:
.subsections_via_symbols

.addrsig
.addrsig_sym _fold_func_low_addr
.addrsig_sym _fold_func_high_addr
.addrsig_sym _fold_func_first_addr
.addrsig_sym _fold_func_second_addr

.text

.globl _fold_func_low_addr
.globl _fold_func_first_addr
.p2align 2
_fold_func_low_addr:
_fold_func_first_addr:
add x0, x0, x0
add x1, x0, x1
add x2, x0, x2
Expand Down Expand Up @@ -383,16 +383,16 @@ _main:
bl _f
bl _g
bl _h
bl _fold_func_low_addr
bl _fold_func_high_addr
bl _fold_func_first_addr
bl _fold_func_second_addr
bl ___nan
bl _objc_msgSend$foo
bl _objc_msgSend$bar
ret

.globl _fold_func_high_addr
.globl _fold_func_second_addr
.p2align 2
_fold_func_high_addr:
_fold_func_second_addr:
add x0, x0, x0
add x1, x0, x1
add x2, x0, x2
Expand Down
44 changes: 44 additions & 0 deletions lld/test/MachO/icf-name-order.s
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@@ -0,0 +1,44 @@
# REQUIRES: aarch64
# RUN: rm -rf %t; split-file %s %t

# This test verifies ICF deterministically chooses root functions based on symbol names
# regardless of input object file order, with all three identical functions being folded together
# and _a chosen as the root due to lexicographic ordering.

# RUN: llvm-mc -filetype=obj -triple=arm64-apple-darwin %t/a.s -o %t/a.o
# RUN: llvm-mc -filetype=obj -triple=arm64-apple-darwin %t/b.s -o %t/b.o
# RUN: llvm-mc -filetype=obj -triple=arm64-apple-darwin %t/c.s -o %t/c.o

# RUN: %lld -dylib -arch arm64 -lSystem -o %t/abc --icf=all -map %t/abc.txt %t/a.o %t/b.o %t/c.o
# RUN: %lld -dylib -arch arm64 -lSystem -o %t/bac --icf=all -map %t/bac.txt %t/b.o %t/a.o %t/c.o
# RUN: %lld -dylib -arch arm64 -lSystem -o %t/cba --icf=all -map %t/cba.txt %t/c.o %t/b.o %t/a.o

# RUN: cat %t/abc.txt | FileCheck %s
# RUN: cat %t/bac.txt | FileCheck %s
# RUN: cat %t/cba.txt | FileCheck %s

# CHECK: Symbols:
# CHECK: [[#%X,ADDR:]] 0x00000008 {{.*}} _a
# CHECK-NEXT: [[#ADDR]] 0x00000000 {{.*}} _b
# CHECK-NEXT: [[#ADDR]] 0x00000000 {{.*}} _c

#--- a.s
.section __TEXT,__text,regular,pure_instructions
.globl _a
_a:
mov x0, 100
ret

#--- b.s
.section __TEXT,__text,regular,pure_instructions
.globl _b
_b:
mov x0, 100
ret

#--- c.s
.section __TEXT,__text,regular,pure_instructions
.globl _c
_c:
mov x0, 100
ret
16 changes: 8 additions & 8 deletions lld/test/MachO/icf-safe-thunks.ll
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Original file line number Diff line number Diff line change
Expand Up @@ -27,8 +27,8 @@
; CHECK-ARM64: _func_call_thunked_1_nomerge:
; CHECK-ARM64-NEXT: stp x29
;
; CHECK-ARM64: _func_call_thunked_2_nomerge:
; CHECK-ARM64-NEXT: _func_call_thunked_2_merge:
; CHECK-ARM64: _func_call_thunked_2_first:
; CHECK-ARM64-NEXT: _func_call_thunked_2_second:
; CHECK-ARM64-NEXT: stp x29
;
; CHECK-ARM64: _call_all_funcs:
Expand All @@ -53,8 +53,8 @@
; CHECK-ARM64-MAP-NEXT: 0x00000000 [ 2] _func_3identical_v2_canmerge
; CHECK-ARM64-MAP-NEXT: 0x00000000 [ 2] _func_3identical_v3_canmerge
; CHECK-ARM64-MAP-NEXT: 0x00000020 [ 2] _func_call_thunked_1_nomerge
; CHECK-ARM64-MAP-NEXT: 0x00000020 [ 2] _func_call_thunked_2_nomerge
; CHECK-ARM64-MAP-NEXT: 0x00000000 [ 2] _func_call_thunked_2_merge
; CHECK-ARM64-MAP-NEXT: 0x00000020 [ 2] _func_call_thunked_2_first
; CHECK-ARM64-MAP-NEXT: 0x00000000 [ 2] _func_call_thunked_2_second
; CHECK-ARM64-MAP-NEXT: 0x00000034 [ 2] _call_all_funcs
; CHECK-ARM64-MAP-NEXT: 0x00000050 [ 2] _take_func_addr
; CHECK-ARM64-MAP-NEXT: 0x00000004 [ 2] _func_2identical_v2
Expand Down Expand Up @@ -93,12 +93,12 @@ ATTR void func_call_thunked_1_nomerge() {
g_val = 77;
}

ATTR void func_call_thunked_2_nomerge() {
ATTR void func_call_thunked_2_first() {
func_2identical_v2();
g_val = 77;
}

ATTR void func_call_thunked_2_merge() {
ATTR void func_call_thunked_2_second() {
func_2identical_v2();
g_val = 77;
}
Expand Down Expand Up @@ -205,14 +205,14 @@ define void @func_call_thunked_1_nomerge() local_unnamed_addr #1 {
}

; Function Attrs: mustprogress nofree noinline norecurse nounwind ssp uwtable(sync)
define void @func_call_thunked_2_nomerge() local_unnamed_addr #1 {
define void @func_call_thunked_2_first() local_unnamed_addr #1 {
tail call void @func_2identical_v2()
store volatile i8 77, ptr @g_val, align 1, !tbaa !4
ret void
}

; Function Attrs: mustprogress nofree noinline norecurse nounwind ssp uwtable(sync)
define void @func_call_thunked_2_merge() local_unnamed_addr #1 {
define void @func_call_thunked_2_second() local_unnamed_addr #1 {
tail call void @func_2identical_v2()
store volatile i8 77, ptr @g_val, align 1, !tbaa !4
ret void
Expand Down
4 changes: 2 additions & 2 deletions lld/test/MachO/map-file.s
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Expand Up @@ -110,8 +110,8 @@
## folded symbols but not folded cstrings; we print both.

# ICF: Symbols:
# ICF-DAG: 0x[[#%X,FOO:]] 0x00000000 [ 3] __ZTIN3foo3bar4MethE
# ICF-DAG: 0x[[#FOO]] 0x00000001 [ 2] _bar
# ICF-DAG: 0x[[#%X,FOO:]] 0x00000001 [ 3] __ZTIN3foo3bar4MethE
# ICF-DAG: 0x[[#FOO]] 0x00000000 [ 2] _bar
# ICF-DAG: 0x[[#%X,HIWORLD:]] 0x0000000E [ 4] literal string: Hello world!\n
# ICF-DAG: 0x[[#%X,HIWORLD]] 0x00000000 [ 4] literal string: Hello world!\n

Expand Down