[WebAssembly] Group rodata into a single output segment

[fitzgen]

Looks good to me!

General question: in the case of an optimized (for size) wasm binary, why not put everything into a single segment instead of a small handful?

[sbc100]

I think you are right about the optimized builds. One thing I thought we might want to preserve is that data objects from the same section should be grouped together contiguously.. but I think we can probably do that while at the same time putting it all in a single segment.

Also, we have post-linking tools such as wasm-opt in binaryen which will actually split up the data segments into smaller ones if they contain runs of zeros. I'm not sure about doing this in the general case since its not clear to me we can rely on the memory we are passed being zero-initialized. In general wasm memory are zero intialized, but the embedding code and reuse/modify memory before wasm init time.

lld/trunk/test/wasm/data-segment-merging.ll

 target triple = "wasm32-unknown-unknown"
 
 @a = hidden global [6 x i8] c"hello\00", align 1
 @b = hidden global [8 x i8] c"goodbye\00", align 1
 @c = hidden global [9 x i8] c"whatever\00", align 1
 @d = hidden global i32 42, align 4
 
+@e = private constant [9 x i8] c"constant\00", align 1
+@f = private constant i8 43, align 4
+
 ; RUN: llc -filetype=obj %s -o %t.data-segment-merging.o
 
 ; RUN: wasm-ld -no-gc-sections --no-entry -o %t.merged.wasm %t.data-segment-merging.o
 ; RUN: obj2yaml %t.merged.wasm | FileCheck %s --check-prefix=MERGE
-; MERGE:       - Type:            DATA
-; MERGE-NEXT:    Segments:
-; MERGE-NEXT:      - SectionOffset:   7
-; MERGE-NEXT:        MemoryIndex:     0
-; MERGE-NEXT:        Offset:
-; MERGE-NEXT:          Opcode:          I32_CONST
-; MERGE-NEXT:          Value:           1024
-; MERGE-NEXT:        Content:         68656C6C6F00676F6F6462796500776861746576657200002A000000
+; MERGE:   - Type:            DATA
+; MERGE:     Segments:
+; MERGE:        Content:         68656C6C6F00676F6F6462796500776861746576657200002A000000
+; MERGE:        Content:         636F6E7374616E74000000002B
+; MERGE-NOT:    Content:
 
 ; RUN: wasm-ld -no-gc-sections --no-entry --no-merge-data-segments -o %t.separate.wasm %t.data-segment-merging.o
 ; RUN: obj2yaml %t.separate.wasm | FileCheck %s --check-prefix=SEPARATE
-; SEPARATE:       - Type:            DATA
-; SEPARATE-NEXT:    Segments:
-; SEPARATE-NEXT:      - SectionOffset:   7
-; SEPARATE-NEXT:        MemoryIndex:     0
-; SEPARATE-NEXT:        Offset:
-; SEPARATE-NEXT:          Opcode:          I32_CONST
-; SEPARATE-NEXT:          Value:           1024
-; SEPARATE-NEXT:        Content:         68656C6C6F00
-; SEPARATE-NEXT:      - SectionOffset:   19
-; SEPARATE-NEXT:        MemoryIndex:     0
-; SEPARATE-NEXT:        Offset:
-; SEPARATE-NEXT:          Opcode:          I32_CONST
-; SEPARATE-NEXT:          Value:           1030
-; SEPARATE-NEXT:        Content:         676F6F6462796500
-; SEPARATE-NEXT:      - SectionOffset:   33
-; SEPARATE-NEXT:        MemoryIndex:     0
-; SEPARATE-NEXT:        Offset:
-; SEPARATE-NEXT:          Opcode:          I32_CONST
-; SEPARATE-NEXT:          Value:           1038
-; SEPARATE-NEXT:        Content:         '776861746576657200'
-; SEPARATE-NEXT:      - SectionOffset:   48
-; SEPARATE-NEXT:        MemoryIndex:     0
-; SEPARATE-NEXT:        Offset:
-; SEPARATE-NEXT:          Opcode:          I32_CONST
-; SEPARATE-NEXT:          Value:           1048
-; SEPARATE-NEXT:        Content:         2A000000
+; SEPARATE:   - Type:            DATA
+; SEPARATE:     Segments:
+; SEPARATE:        Content:         68656C6C6F00
+; SEPARATE:        Content:         676F6F6462796500
+; SEPARATE:        Content:         '776861746576657200'
+; SEPARATE:        Content:         2A000000
+; SEPARATE:        Content:         636F6E7374616E7400
+; SEPARATE:        Content:         2B
+; SEPARATE-NOT:    Content:

lld/trunk/wasm/Writer.cpp

static StringRef getOutputDataSegmentName(StringRef Name) {
   if (!Config->MergeDataSegments)
     return Name;
   if (Name.startswith(".text."))
     return ".text";
   if (Name.startswith(".data."))
     return ".data";
   if (Name.startswith(".bss."))
     return ".bss";
+  if (Name.startswith(".rodata."))
+    return ".rodata";
   return Name;
 }
 
 void Writer::createOutputSegments() {
   for (ObjFile *File : Symtab->ObjectFiles) {
     for (InputSegment *Segment : File->Segments) {
       if (!Segment->Live)
         continue;