Wasm32: Optimize small struct returns as integer values for better ABI efficiency

[kassane]

The current implementation of LDC2 (latest-CI) is using the struct return (sret) mechanism for all struct returns, including small structs that could fit in a single register. This approach, while general and correct, may lead to suboptimal performance for small structs, especially on targets like wasm32-emscripten.

Note: They both have the same configuration, increasing the -sSTACK_SIZE=512KB.

full LLVM-IR: https://gist.github.com/kassane/bbae501db913ccceaa9b600b1e0930b7 (ReleaseMode)

LLVM-IR (sg_make_buffer)

Based code:

// sokol_gfx.h
typedef struct sg_buffer        { uint32_t id; } sg_buffer;
inline sg_buffer sg_make_buffer(const sg_buffer_desc& desc) { return sg_make_buffer(&desc); }
SOKOL_GFX_API_DECL sg_buffer sg_make_buffer(const sg_buffer_desc* desc);

D binding

// sokol/gfx.d

extern(C)
struct Buffer {
    uint id = 0;
}

extern(C) Buffer sg_make_buffer(const BufferDesc *);

cube.zig

cube.d

; Function Attrs: noredzone nosanitize_coverage nounwind skipprofile uwtable define dso_local void @init() #0 !dbg !632 { ; [...] %7 = call i32 @sg_make_buffer(ptr nonnull readonly align 4 %0), !dbg !887, !noalias !888 %8 = call i32 @sg_make_buffer(ptr nonnull readonly align 4 %0), !dbg !895, !noalias !896 ; [...] ; Function Attrs: noredzone nounwind uwtable declare i32 @sg_make_buffer(ptr readonly align 4) local_unnamed_addr #7

; [#uses = 1] define void @init() #2 { ; [...] call void @sg_make_buffer(ptr noalias nonnull sret(%sokol.gfx.Buffer) align 4 %.sret_tmp, ptr nonnull %vbufd) #2 call void @sg_make_buffer(ptr noalias nonnull sret(%sokol.gfx.Buffer) align 4 %.sret_tmp1, ptr nonnull %ibufd) #2 ; [...] ; [#uses = 0] define void @_D5sokol3gfx10makeBufferFNbNiNeMKSQBgQBd10BufferDescZSQCaQBx6Buffer(ptr noalias sret(%sokol.gfx.Buffer) align 4 %.sret_arg, ptr dereferenceable(56) %desc) local_unnamed_addr #2 { tail call void @sg_make_buffer(ptr noalias sret(%sokol.gfx.Buffer) align 4 %.sret_arg, ptr nonnull %desc) #2 ret void } ; [#uses = 3] declare void @sg_make_buffer(ptr noalias sret(%sokol.gfx.Buffer) align 4, ptr) local_unnamed_addr #2

cube output

[sg][error][id:5][line:8473] 
cube.js:63:183
[sg][info][id:5][line:8474] 
	ERROR: 1:1: '' : syntax error
cube.js:63:243
[sg][error][id:5][line:8473] 
cube.js:63:183
[sg][info][id:5][line:8474] 
	ERROR: 1:1: '' : syntax error
cube.js:63:243
onerror: RuntimeError: index out of bounds cube.html:1:1053
Uncaught RuntimeError: index out of bounds
    c http://localhost:6931/cube.js:40
    Qa http://localhost:6931/cube.js:40
    _main http://localhost:6931/cube.js:65
    a http://localhost:6931/cube.js:67
    Ec http://localhost:6931/cube.js:67
    setTimeout handler*Ec http://localhost:6931/cube.js:67
    Dc http://localhost:6931/cube.js:66
    a http://localhost:6931/cube.js:64
    <anonymous> http://localhost:6931/cube.js:64
    promise callback*Ba/< http://localhost:6931/cube.js:6
    promise callback*Ba http://localhost:6931/cube.js:6
    Z http://localhost:6931/cube.js:64
    <anonymous> http://localhost:6931/cube.js:64
cube.wasm:33396:1
WebGL warning: bufferData: target: Invalid enum value 0 (Did you typo `gl.SOMETHINGG` and pass `undefined`?)
Error: Promised response from onMessage listener went out of scope ExtensionMessagingService.js:89:34

ldc2-flags to llvmIR

$ ldmd2 -w -c -preview=all --output-ll -O3 -boundscheck=off -vcolumns -cache=/home/kassane/sokol-d/.zig-cache/o/2e1a6a13eb8fd730ff21e1a4e29b9dfd -disable-verify -Hkeep-all-bodies -i -I./src /home/kassane/sokol-d/src/examples/cube.d --d-version=CarelessAlocation -L-allow-undefined -vdmd -Xcc=-v -P-I/home/kassane/.cache/zig/p/12208a21e4a62ced46244c44c6215ff99c03c0d384ec3afea04b32c4b8ecb1baa41a/upstream/emscripten/cache/sysroot/include -Xcc=-DIMPL -Xcc=-DSOKOL_GLES3 --flto=full -mtriple=wasm32-unknown-emscripten -mcpu=generic -betterC --verrors-context

Could it be possible to simplify the function's return rather than sret?

Reference

• wasm32: some examples get RuntimeError kassane/sokol-d#22

[kassane]

Source in C++ (extern"C"): https://godbolt.org/z/a1ra1sxf7 to wasm32

define hidden noundef i32 @main() {
entry:
 ; [...]
  %call = call i32 @sg_make_buffer(ptr noundef nonnull %b)
  ;[...]
}
; [...]
declare i32 @sg_make_buffer(ptr noundef) local_unnamed_addr #3

[kinke]

Analogous to #4741, we'd need a specific TargetABI implementation for wasm, using https://github.com/WebAssembly/tool-conventions/blob/main/BasicCABI.md#function-arguments-and-return-values as reference.

Edit: The main interesting point being that only these structs would need to be rewritten and passed as a primitive type:

Any struct or union that recursively (including through nested structs, unions, and arrays) contains just a single scalar value and is not specified to have greater than natural alignment.