[lld] [lld][WebAssembly] Skip BSS when combining data segments (PR #127735)
Sam Clegg via llvm-commits
llvm-commits at lists.llvm.org
Thu Feb 20 14:08:15 PST 2025
https://github.com/sbc100 updated https://github.com/llvm/llvm-project/pull/127735
>From 6719507ed8df6bbf4fc6269e80a0f06c63721a23 Mon Sep 17 00:00:00 2001
From: Sam Clegg <sbc at chromium.org>
Date: Tue, 18 Feb 2025 14:07:04 -0800
Subject: [PATCH] [lld][WebAssembly] Honor requiredInBinary when combining data
segments
In most circumstances BSS segments are not required in the output binary
but combineOutputSegments was erroneously including them. This meant
that PIC binaries were including the BSS data as zero in the binary.
Fixes: https://github.com/emscripten-core/emscripten/issues/23683
---
lld/test/wasm/data-segments.ll | 37 +++++++++++++++++++++++++++++++---
lld/wasm/OutputSections.cpp | 3 ++-
lld/wasm/Writer.cpp | 7 ++++++-
3 files changed, 42 insertions(+), 5 deletions(-)
diff --git a/lld/test/wasm/data-segments.ll b/lld/test/wasm/data-segments.ll
index 79f1d384919d9..6c401c4873910 100644
--- a/lld/test/wasm/data-segments.ll
+++ b/lld/test/wasm/data-segments.ll
@@ -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
@@ -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
@@ -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
@@ -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:
diff --git a/lld/wasm/OutputSections.cpp b/lld/wasm/OutputSections.cpp
index 95f7ecc29de6b..d679d1e676479 100644
--- a/lld/wasm/OutputSections.cpp
+++ b/lld/wasm/OutputSections.cpp
@@ -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
diff --git a/lld/wasm/Writer.cpp b/lld/wasm/Writer.cpp
index 76e38f548157c..7770bdcfc1f16 100644
--- a/lld/wasm/Writer.cpp
+++ b/lld/wasm/Writer.cpp
@@ -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)
@@ -1100,7 +1105,7 @@ void Writer::combineOutputSegments() {
}
}
- segments = {combined};
+ segments = newSegments;
}
static void createFunction(DefinedFunction *func, StringRef bodyContent) {
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