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[lld][WebAssembly] Skip BSS when combining data segments #127735

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Feb 20, 2025
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[lld][WebAssembly] Skip BSS when combining data segments #127735

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37 changes: 34 additions & 3 deletions lld/test/wasm/data-segments.ll
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Original file line number Diff line number Diff line change
Expand Up @@ -17,6 +17,11 @@
; RUN: wasm-ld -mwasm64 -no-gc-sections --no-entry %t.bulk-mem64.o -o %t.bulk-mem64.wasm
; RUN: obj2yaml %t.bulk-mem64.wasm | FileCheck %s --check-prefixes ACTIVE,ACTIVE64

;; In -pie mode segments are combined into one active segment.
; RUN: wasm-ld --experimental-pic --import-memory -pie -no-gc-sections --no-entry %t.atomics.bulk-mem.pic.o -o %t.pic.wasm
; RUN: obj2yaml %t.pic.wasm | FileCheck %s --check-prefixes ACTIVE-PIC
; RUN: llvm-objdump --disassemble-symbols=__wasm_call_ctors,__wasm_init_memory --no-show-raw-insn --no-leading-addr %t.pic.wasm | FileCheck %s --check-prefixes PIC-NON-SHARED-DIS

;; atomics, bulk memory, shared memory => passive segments
; RUN: wasm-ld -no-gc-sections --no-entry --shared-memory --max-memory=131072 %t.atomics.bulk-mem.o -o %t.atomics.bulk-mem.wasm
; RUN: obj2yaml %t.atomics.bulk-mem.wasm | FileCheck %s --check-prefix PASSIVE
Expand All @@ -28,9 +33,9 @@
; RUN: llvm-objdump --disassemble-symbols=__wasm_call_ctors,__wasm_init_memory --no-show-raw-insn --no-leading-addr %t.atomics.bulk-mem64.wasm | FileCheck %s --check-prefixes DIS,NOPIC-DIS -DPTR=i64

;; Also test in combination with PIC/pie
; RUN: wasm-ld --experimental-pic -pie -no-gc-sections --no-entry --shared-memory --max-memory=131072 %t.atomics.bulk-mem.pic.o -o %t.pic.wasm
; RUN: obj2yaml %t.pic.wasm | FileCheck %s --check-prefixes PASSIVE-PIC,PASSIVE32-PIC
; RUN: llvm-objdump --disassemble-symbols=__wasm_call_ctors,__wasm_init_memory --no-show-raw-insn --no-leading-addr %t.pic.wasm | FileCheck %s --check-prefixes DIS,PIC-DIS -DPTR=i32
; RUN: wasm-ld --experimental-pic -pie -no-gc-sections --no-entry --shared-memory --max-memory=131072 %t.atomics.bulk-mem.pic.o -o %t.shared.pic.wasm
; RUN: obj2yaml %t.shared.pic.wasm | FileCheck %s --check-prefixes PASSIVE-PIC,PASSIVE32-PIC
; RUN: llvm-objdump --disassemble-symbols=__wasm_call_ctors,__wasm_init_memory --no-show-raw-insn --no-leading-addr %t.shared.pic.wasm | FileCheck %s --check-prefixes DIS,PIC-DIS -DPTR=i32

;; Also test in combination with PIC/pie + wasm64
; RUN: wasm-ld -mwasm64 --experimental-pic -pie -no-gc-sections --no-entry --shared-memory --max-memory=131072 %t.atomics.bulk-mem.pic-mem64.o -o %t.pic-mem64.wasm
Expand Down Expand Up @@ -76,6 +81,20 @@
; ACTIVE-NEXT: - Index: 0
; ACTIVE-NEXT: Name: __wasm_call_ctors

;; In ACTIVE-PIC mode the memory is imported which means all data segments
;; (except BSS) are combined in the single one.
;; BSS is not included here, and instead initialized using `memory.init` in
;; `__wasm_init_memory`

; ACTIVE-PIC: - Type: DATA
; ACTIVE-PIC-NEXT: Segments:
; ACTIVE-PIC-NEXT: - SectionOffset: 6
; ACTIVE-PIC-NEXT: InitFlags: 0
; ACTIVE-PIC-NEXT: Offset:
; ACTIVE-PIC-NEXT: Opcode: GLOBAL_GET
; ACTIVE-PIC-NEXT: Index: 1
; ACTIVE-PIC-NEXT: Content: 63000000636F6E7374616E74000000002B00000068656C6C6F00676F6F646279650000002A000000

; PASSIVE-LABEL: - Type: START
; PASSIVE-NEXT: StartFunction: 2
; PASSIVE-LABEL: - Type: DATACOUNT
Expand Down Expand Up @@ -151,6 +170,18 @@
; PASSIVE-PIC-NEXT: - Index: 2
; PASSIVE-PIC-NEXT: Name: __wasm_init_memory

;; For the non-shared PIC case the __wasm_init_memory only deals with BSS since
;; all other segments are active
; PIC-NON-SHARED-DIS: <__wasm_init_memory>:
; PIC-NON-SHARED-DIS-EMPTY:
; PIC-NON-SHARED-DIS-NEXT: i32.const 40
; PIC-NON-SHARED-DIS-NEXT: global.get 1
; PIC-NON-SHARED-DIS-NEXT: i32.add
; PIC-NON-SHARED-DIS-NEXT: i32.const 0
; PIC-NON-SHARED-DIS-NEXT: i32.const 10000
; PIC-NON-SHARED-DIS-NEXT: memory.fill 0
; PIC-NON-SHARED-DIS-NEXT: end

;; no data relocations.
; DIS-LABEL: <__wasm_call_ctors>:
; DIS-EMPTY:
Expand Down
3 changes: 2 additions & 1 deletion lld/wasm/OutputSections.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -101,7 +101,8 @@ void DataSection::finalizeContents() {
});
#ifndef NDEBUG
unsigned activeCount = llvm::count_if(segments, [](OutputSegment *segment) {
return (segment->initFlags & WASM_DATA_SEGMENT_IS_PASSIVE) == 0;
return segment->requiredInBinary() &&
(segment->initFlags & WASM_DATA_SEGMENT_IS_PASSIVE) == 0;
});
#endif

Expand Down
7 changes: 6 additions & 1 deletion lld/wasm/Writer.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1081,7 +1081,12 @@ void Writer::combineOutputSegments() {
return;
OutputSegment *combined = make<OutputSegment>(".data");
combined->startVA = segments[0]->startVA;
std::vector<OutputSegment *> newSegments = {combined};
for (OutputSegment *s : segments) {
if (!s->requiredInBinary()) {
newSegments.push_back(s);
continue;
}
bool first = true;
for (InputChunk *inSeg : s->inputSegments) {
if (first)
Expand All @@ -1100,7 +1105,7 @@ void Writer::combineOutputSegments() {
}
}

segments = {combined};
segments = newSegments;
}

static void createFunction(DefinedFunction *func, StringRef bodyContent) {
Expand Down
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