Commit d66c61a2cf69665223815a1a12a1f93b30b99571 | zig

author	Luuk de Gram <luuk@degram.dev> 2022-04-13 22:14:48+02:00
committer	Luuk de Gram <luuk@degram.dev> 2022-04-14 22:53:13+02:00
log	d66c61a2cf69665223815a1a12a1f93b30b99571
tree	2f969fb2733eadc8ae16088ce98b0b19a5841ce8
parent	cf37101108e56412e958a8b597bae84649893b6f

wasm-linker: Prevent overalignment for segments

Previously, the data segments were being aligned twice.
This caused us to overalign the segment and therefore allocate a much larger
size for each segment than was required. This fix ensures we align and set the size
just once, ensuring semantically correct binaries as well as smaller binaries.

2 files changed, 7 insertions(+), 15 deletions(-)

src
- link
  - Wasm.zig
  - Wasm
    Object.zig

src/link/Wasm.zig+7-10

...	@@ -962,11 +962,6 @@ fn parseAtom(self: Wasm, atom: Atom, kind: Kind) !void {	...	@@ -962,11 +962,6 @@ fn parseAtom(self: Wasm, atom: Atom, kind: Kind) !void {
962		962
963	const segment: *Segment = &self.segments.items[final_index];	963	const segment: *Segment = &self.segments.items[final_index];
964	segment.alignment = std.math.max(segment.alignment, atom.alignment);	964	segment.alignment = std.math.max(segment.alignment, atom.alignment);
965	segment.size = std.mem.alignForwardGeneric(
966	u32,
967	std.mem.alignForwardGeneric(u32, segment.size, atom.alignment) + atom.size,
968	segment.alignment,
969	);
970		965
971	if (self.atoms.getPtr(final_index)) \|last\| {	966	if (self.atoms.getPtr(final_index)) \|last\| {
972	last.*.next = atom;	967	last.*.next = atom;
...	@@ -978,9 +973,10 @@ fn parseAtom(self: Wasm, atom: Atom, kind: Kind) !void {	...	@@ -978,9 +973,10 @@ fn parseAtom(self: Wasm, atom: Atom, kind: Kind) !void {
978	}	973	}
979		974
980	fn allocateAtoms(self: *Wasm) !void {	975	fn allocateAtoms(self: *Wasm) !void {
981	var it = self.atoms.valueIterator();	976	var it = self.atoms.iterator();
982	while (it.next()) \|current_atom\| {	977	while (it.next()) \|entry\| {
983	var atom: Atom = current_atom..getFirst();	978	const segment = &self.segments.items[entry.key_ptr.*];
		979	var atom: Atom = entry.value_ptr..getFirst();
984	var offset: u32 = 0;	980	var offset: u32 = 0;
985	while (true) {	981	while (true) {
986	offset = std.mem.alignForwardGeneric(u32, offset, atom.alignment);	982	offset = std.mem.alignForwardGeneric(u32, offset, atom.alignment);
...	@@ -996,6 +992,7 @@ fn allocateAtoms(self: *Wasm) !void {	...	@@ -996,6 +992,7 @@ fn allocateAtoms(self: *Wasm) !void {
996	self.symbol_atom.putAssumeCapacity(atom.symbolLoc(), atom); // Update atom pointers	992	self.symbol_atom.putAssumeCapacity(atom.symbolLoc(), atom); // Update atom pointers
997	atom = atom.next orelse break;	993	atom = atom.next orelse break;
998	}	994	}
		995	segment.size = std.mem.alignForwardGeneric(u32, offset, segment.alignment);
999	}	996	}
1000	}	997	}
1001		998
...	@@ -1566,8 +1563,8 @@ pub fn flushModule(self: Wasm, comp: Compilation) !void {	...	@@ -1566,8 +1563,8 @@ pub fn flushModule(self: Wasm, comp: Compilation) !void {
1566	try self.objects.items[object_index].parseIntoAtoms(self.base.allocator, object_index, self);	1563	try self.objects.items[object_index].parseIntoAtoms(self.base.allocator, object_index, self);
1567	}	1564	}
1568		1565
1569	try self.setupMemory();
1570	try self.allocateAtoms();	1566	try self.allocateAtoms();
		1567	try self.setupMemory();
1571	self.mapFunctionTable();	1568	self.mapFunctionTable();
1572	try self.mergeSections();	1569	try self.mergeSections();
1573	try self.mergeTypes();	1570	try self.mergeTypes();
...	@@ -1832,7 +1829,7 @@ pub fn flushModule(self: Wasm, comp: Compilation) !void {	...	@@ -1832,7 +1829,7 @@ pub fn flushModule(self: Wasm, comp: Compilation) !void {
1832	segment_count += 1;	1829	segment_count += 1;
1833	const atom_index = entry.value_ptr.*;	1830	const atom_index = entry.value_ptr.*;
1834	var atom: Atom = self.atoms.getPtr(atom_index).?..getFirst();	1831	var atom: Atom = self.atoms.getPtr(atom_index).?..getFirst();
1835	var segment = self.segments.items[atom_index];	1832	const segment = self.segments.items[atom_index];
1836		1833
1837	// flag and index to memory section (currently, there can only be 1 memory section in wasm)	1834	// flag and index to memory section (currently, there can only be 1 memory section in wasm)
1838	try leb.writeULEB128(writer, @as(u32, 0));	1835	try leb.writeULEB128(writer, @as(u32, 0));

src/link/Wasm/Object.zig-5

 @@ -865,11 +865,6 @@ pub fn parseIntoAtoms(self: *Object, gpa: Allocator, object_index: u16, wasm_bin
         const segment: *Wasm.Segment = &wasm_bin.segments.items[final_index];
         segment.alignment = std.math.max(segment.alignment, atom.alignment);
-        segment.size = std.mem.alignForwardGeneric(
-            u32,
-            std.mem.alignForwardGeneric(u32, segment.size, atom.alignment) + atom.size,
-            segment.alignment,
-        );
         if (wasm_bin.atoms.getPtr(final_index)) |last| {
             last.*.next = atom;