(feat) Static analysis, pull only what is needed to optimize build for size (and speed) #1588

guest271314 · 2026-07-18T14:40:44Z

guest271314
Jul 18, 2026

Let's say you start with source code

const u8 = new Uint8Array(4096);

We can see we don't need a whole bunch (or all) of ECMAScript interfaces for that code to compile/execute.

We don't need RegExp, String, Date, Proxy, Promise, and many other standard ECMA-262 interfaces. So, for a generic build a lot of the engine built won't be used to execute the specific instruction(s) in the source code.

The execution environment is not necessarily a browser, so we are not concerned with JIT, or having any/all interfaces available for whatever a Web site might have by way of scripting.

We are not concerned with passing every test in test262 just because it's there. We are only concerned with creating the engine to fulfill what the source code says is needed to complete the instruction(s).

Additionally, starting out from the outset with the idea that WASM is a target for the engine, too.

Feasibility?

Answered by bnoordhuis

Jul 19, 2026

That kind of analysis is not an ambition we have.

With that out of the way: you can strip the standard environment down to almost nothing by replacing JS_NewContext() with JS_NewContextRaw() + JS_AddIntrinsicXXX() calls. That's a manual process though.

View full answer

bnoordhuis · 2026-07-19T19:53:53Z

bnoordhuis
Jul 19, 2026
Maintainer

That kind of analysis is not an ambition we have.

With that out of the way: you can strip the standard environment down to almost nothing by replacing JS_NewContext() with JS_NewContextRaw() + JS_AddIntrinsicXXX() calls. That's a manual process though.

23 replies

guest271314 Jul 20, 2026
Author

Looks like you should keep libregexp.c and trust LTO will realize and strip it out.

Yeah. I went ahead and downloaded gcc 15.3.0. After hours it still had not completed building before I CTRL+C'ed the shell. Then I remembered I have zig - and rustc. So I compiled the native executable with zig 0.16.0. That got the native executable down to 582.1 KiB

CC="/home/user/bin/zig cc"
# SYSROOT="$WASI_SDK_PATH/share/wasi-sysroot"

if [ ! -d "build" ]; then
    echo "build directory doesn't exist, create..."
    mkdir build
else
    echo "create build directory."
fi

cp -r qjs.c quickjs
OPTIMIZATIONS="-Oz -flto"

DEFINES="-D_GNU_SOURCE -include alloca.h -pthread"

CFLAGS="-std=c23 $OPTIMIZATIONS $DEFINES -I."

$CC $CFLAGS \
  -Wl,--strip-all \
  -Wl,--gc-sections \
  ./quickjs/quickjs.c \
  ./quickjs/qjs.c \
  ./quickjs/dtoa.c \
  ./quickjs/libunicode.c \
  ./quickjs/libregexp.c \
  ./quickjs/quickjs-libc.c \
  -o qjs-embedded \

guest271314 Jul 20, 2026
Author

Looks like you should keep libregexp.c and trust LTO will realize and strip it out.

So, right now looks like the 550 KiB mark is what I'm trying to get below. If we could get to around 250 KiB for WASM and native builds that would be a feat.

bnoordhuis Jul 20, 2026
Maintainer

Besides LTO you can also try -ffunction-sections -fdata-sections in combination with the --gc-sections linker flag (and maybe --print-gc-sections so you can see what's getting removed.)

The linker should, in principle, be smart enough to delete everything that isn't reachable from your program's main() function.

guest271314 Jul 21, 2026
Author

Here's the zig cc 0.16.0 version, that produces a 580.8 KiB WASM binary. Doesn't print the diagnositic data.

WASI_SDK_PATH="$PWD/wasi-sdk-34.0-rc.1-x86_64-linux"
CC="/home/user/bin/zig cc -target wasm32-wasi" #"$WASI_SDK_PATH/bin/clang"
SYSROOT="$WASI_SDK_PATH/share/wasi-sysroot"

if [ ! -d "build" ]; then
    echo "build directory doesn't exist, create..."
    mkdir build
else
    echo "create build directory."
fi


OPTIMIZATIONS="-Oz -flto -ffunction-sections -fdata-sections"

DEFINES="-DCONFIG_VERSION=\"0.14.0\" -D_GNU_SOURCE -D_WASI_EMULATED_SIGNAL"

CFLAGS="-std=c23 --sysroot=$SYSROOT $OPTIMIZATIONS $DEFINES -I." # --sysroot=$SYSROOT $OPTIMIZATIONS

echo "Compiling objects..."

$CC $CFLAGS -c ./quickjs/quickjs.c -o build/quickjs.o
$CC $CFLAGS -c ./quickjs/qjs.c -o build/qjs.o
$CC $CFLAGS -c ./quickjs/dtoa.c -o build/dtoa.o
$CC $CFLAGS -c ./quickjs/libunicode.c -o build/libunicode.o
$CC $CFLAGS -c ./quickjs/libregexp.c -o build/libregexp.o
$CC $CFLAGS -Denviron=__wasilibc_environ -c ./quickjs/quickjs-libc.c -o build/quickjs-libc.o

echo "Linking..."

$CC --sysroot=$SYSROOT $OPTIMIZATIONS \
  -DCONFIG_WASI=y \
  -Wl,--strip-all \
  -Wl,--gc-sections \
  -Wl,--print-gc-sections \
  build/quickjs.o \
  build/quickjs-libc.o \
  build/qjs.o \
  build/dtoa.o \
  build/libunicode.o \
  build/libregexp.o \
  -lwasi-emulated-signal \
  -lwasi-emulated-mman \
  -o qjs-wasi.wasm

Nothing changed with wasi-sdk-34.0-rc.1-x86_64-linux version, 550 KiB, does print diagnostic data.

cp -r qjs.c quickjs
WASI_SDK_PATH="$PWD/wasi-sdk-34.0-rc.1-x86_64-linux"
CC="$WASI_SDK_PATH/bin/clang"
SYSROOT="$WASI_SDK_PATH/share/wasi-sysroot"

if [ ! -d "build" ]; then
    echo "build directory doesn't exist, create..."
    mkdir build
else
    echo "create build directory."
fi


OPTIMIZATIONS="-Oz -flto -ffunction-sections -fdata-sections"

DEFINES="-DCONFIG_VERSION=\"0.14.0\" -D_GNU_SOURCE -D_WASI_EMULATED_SIGNAL"

CFLAGS="-std=c23 --sysroot=$SYSROOT $OPTIMIZATIONS $DEFINES -I." # --sysroot=$SYSROOT $OPTIMIZATIONS

echo "Compiling objects..."

$CC $CFLAGS -c ./quickjs/quickjs.c -o build/quickjs.o
$CC $CFLAGS -c ./quickjs/qjs.c -o build/qjs.o
$CC $CFLAGS -c ./quickjs/dtoa.c -o build/dtoa.o
$CC $CFLAGS -c ./quickjs/libunicode.c -o build/libunicode.o
$CC $CFLAGS -c ./quickjs/libregexp.c -o build/libregexp.o
$CC $CFLAGS -Denviron=__wasilibc_environ -c ./quickjs/quickjs-libc.c -o build/quickjs-libc.o

echo "Linking..."

$CC --target=wasm32-wasip1 --sysroot=$SYSROOT $OPTIMIZATIONS \
  -DCONFIG_WASI=y \
  -Wl,--strip-all \
  -Wl,--gc-sections \
  -Wl,--print-gc-sections \
  build/quickjs.o \
  build/quickjs-libc.o \
  build/qjs.o \
  build/dtoa.o \
  build/libunicode.o \
  build/libregexp.o \
  -lwasi-emulated-signal \
  -lwasi-emulated-mman \
  -o qjs-wasi.wasm

build.sh
wasi-sdk directory already exists. Skipping download.
quickjs repository already exists. Skipping clone.
create build directory.
Compiling objects...
Linking...
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removing unused section qjs-wasi.wasm.lto.o:(.rodata.atanlo.887)
removing unused section qjs-wasi.wasm.lto.o:(.rodata.atanhi.888)
removing unused section qjs-wasi.wasm.lto.o:(.rodata.atanlo.889)
removing unused section qjs-wasi.wasm.lto.o:(.rodata.exp10.p10)
removing unused section qjs-wasi.wasm.lto.o:(.rodata.exp10f.p10)
removing unused section qjs-wasi.wasm.lto.o:(.rodata.tbl)
removing unused section qjs-wasi.wasm.lto.o:(.rodata.eps)
removing unused section qjs-wasi.wasm.lto.o:(.rodata.__log2f_data)
removing unused section qjs-wasi.wasm.lto.o:(.rodata.__logf_data)
removing unused section qjs-wasi.wasm.lto.o:(.rodata.__exp2f_data)
removing unused section qjs-wasi.wasm.lto.o:(.rodata.__powf_log2_data)
removing unused section qjs-wasi.wasm.lto.o:(.rodata.__rsqrt_tab)
removing unused section qjs-wasi.wasm.lto.o:(.rodata.fact)
removing unused section qjs-wasi.wasm.lto.o:(.rodata.Snum)
removing unused section qjs-wasi.wasm.lto.o:(.rodata.Sden)
Done compiling qjs-wasi.wasm with index.js embedded.

Compiling to native executable using zig cc produces a 582.1 KiB executable

CC="/home/user/bin/zig cc"
# SYSROOT="$WASI_SDK_PATH/share/wasi-sysroot"

cp -r qjs.c quickjs
OPTIMIZATIONS="-Oz -flto -ffunction-sections -fdata-sections"

DEFINES="-D_GNU_SOURCE -include alloca.h -pthread"

CFLAGS="-std=c23 $OPTIMIZATIONS $DEFINES -I."

$CC $CFLAGS \
  -Wl,--strip-all \
  -Wl,--gc-sections \
  -Wl,--print-gc-sections \
  ./quickjs/quickjs.c \
  ./quickjs/qjs.c \
  ./quickjs/dtoa.c \
  ./quickjs/libunicode.c \
  ./quickjs/libregexp.c \
  ./quickjs/quickjs-libc.c \
  -o qjs-embedded-zig-cc \

 ./build-native-executable.sh
quickjs repository already exists. Skipping clone.
create build directory.
removing unused section /usr/lib/gcc/x86_64-linux-gnu/14/../../../x86_64-linux-gnu/crt1.o:(.rodata.cst4)
removing unused section /usr/lib/gcc/x86_64-linux-gnu/14/../../../x86_64-linux-gnu/crt1.o:(.data)
removing unused section /usr/lib/gcc/x86_64-linux-gnu/14/../../../x86_64-linux-gnu/crt1.o:(.bss)
removing unused section /usr/lib/gcc/x86_64-linux-gnu/14/../../../x86_64-linux-gnu/crti.o:(.text)
removing unused section /usr/lib/gcc/x86_64-linux-gnu/14/../../../x86_64-linux-gnu/crti.o:(.data)
removing unused section /usr/lib/gcc/x86_64-linux-gnu/14/../../../x86_64-linux-gnu/crti.o:(.bss)
removing unused section /usr/lib/x86_64-linux-gnu/libc_nonshared.a(atexit.oS):(.text)
removing unused section /usr/lib/x86_64-linux-gnu/libc_nonshared.a(atexit.oS):(.data)
removing unused section /usr/lib/x86_64-linux-gnu/libc_nonshared.a(atexit.oS):(.bss)
removing unused section /usr/lib/gcc/x86_64-linux-gnu/14/../../../x86_64-linux-gnu/crtn.o:(.text)
removing unused section /usr/lib/gcc/x86_64-linux-gnu/14/../../../x86_64-linux-gnu/crtn.o:(.data)
removing unused section /usr/lib/gcc/x86_64-linux-gnu/14/../../../x86_64-linux-gnu/crtn.o:(.bss)
removing unused section qjs-embedded-zig-cc.lto.o:(.text)
removing unused section qjs-embedded-zig-cc.lto.o:(.rodata..L__const.JS_AtomIsNumericIndex1.nfinity16)
Done compiling qjs-embedded with index.js embedded.

clang version 20.0.0git (https://github.com/llvm/llvm-project.git ae719f07562a2eb74f620b1f6d798d6507760514) Target: x86_64-unknown-linux-gnu Thread model: posix. Since I evidently didn't build the llvm-project with LTO optimizations they are not available for the built clang, which produces an 627.1 KiB executable - without -flto

CC="/home/user/bin/clang"
# SYSROOT="$WASI_SDK_PATH/share/wasi-sysroot"

OPTIMIZATIONS="-Oz -ffunction-sections -fdata-sections"

DEFINES="-D_GNU_SOURCE -include alloca.h -pthread"

CFLAGS="-std=c23 $OPTIMIZATIONS $DEFINES -I."

echo "Compiling objects..."

$CC $CFLAGS -c ./quickjs/quickjs.c -o build/quickjs.o
$CC $CFLAGS -c ./quickjs/qjs.c -o build/qjs.o
$CC $CFLAGS -c ./quickjs/dtoa.c -o build/dtoa.o
$CC $CFLAGS -c ./quickjs/libunicode.c -o build/libunicode.o
$CC $CFLAGS -c ./quickjs/libregexp.c -o build/libregexp.o
$CC $CFLAGS -c ./quickjs/quickjs-libc.c -o build/quickjs-libc.o

echo "Linking..."

$CC $OPTIMIZATIONS \
  -Wl,--strip-all \
  -Wl,--gc-sections \
  build/quickjs.o \
  build/quickjs-libc.o \
  build/qjs.o \
  build/dtoa.o \
  build/libunicode.o \
  build/libregexp.o \
  -o qjs-embedded-clang \
  -lm -pthread

If I do something special for gcc (Debian 14.2.0-19) 14.2.0 for that #embed the result here is 595.1 KiB

const char* str = "import*as r from\"qjs:std\";const O=()=>{const t=new Uint32Array(1);r.in.read(t.buffer,0,4);const l=new Uint8Array(t[0]),h=r.in.read(l.buffer,0,l.length);return l},U=(t)=>{const l=44,h=91,i=93,y=1048576;if(t.length<=y){const n=new Uint8Array(4);n[0]=t.length>>0&255;n[1]=t.length>>8&255;n[2]=t.length>>16&255;n[3]=t.length>>24&255;const f=new Uint8Array(4+t.length);f.set(n,0);f.set(t,4);r.out.write(f.buffer,0,f.length);r.out.flush();return}let s=0;while(s<t.length){let n,f=s+y-8;if(f>=t.length)n=t.length;else{n=t.indexOf(l,f);if(n===-1)n=t.length}const u=t.subarray(s,n),c=u[0],g=u[u.length-1];let p=null,a=null;if(c===h&&g!==i)a=i;else if(c===l){p=h;if(g!==i)a=i}let x=u.length,B=4;const w=p!==null,b=a!==null;let A=0;if(c===l){A=1;x-=1}const C=4+(w?1:0)+x+(b?1:0),e=new Uint8Array(C),d=C-4;e[0]=d>>0&255;e[1]=d>>8&255;e[2]=d>>16&255;e[3]=d>>24&255;let o=4;if(w){e[o]=p;o++}else if(c===l){e[o]=h;o++}e.set(u.subarray(A),o);o+=x;if(b)e[o]=a;r.out.write(e.buffer,0,e.length);r.out.flush();r.gc();s=n}},E=()=>{while(!0){const t=O();U(t)}};try{E()}catch(t){r.exit(0)}";
//const char* str = (const char[]) {
//  #embed "../index.js"
//  , 0 // Null terminator
//};

CC="gcc"
# SYSROOT="$WASI_SDK_PATH/share/wasi-sysroot"

OPTIMIZATIONS="-Oz -flto -ffunction-sections -fdata-sections"

DEFINES="-D_GNU_SOURCE -include alloca.h -pthread"

CFLAGS="-std=c23 $OPTIMIZATIONS $DEFINES -I."

echo "Compiling objects..."

$CC $CFLAGS -c ./quickjs/quickjs.c -o build/quickjs.o
$CC $CFLAGS -c ./quickjs/qjs.c -o build/qjs.o
$CC $CFLAGS -c ./quickjs/dtoa.c -o build/dtoa.o
$CC $CFLAGS -c ./quickjs/libunicode.c -o build/libunicode.o
$CC $CFLAGS -c ./quickjs/libregexp.c -o build/libregexp.o
$CC $CFLAGS -c ./quickjs/quickjs-libc.c -o build/quickjs-libc.o

echo "Linking..."

$CC $OPTIMIZATIONS \
  -Wl,--strip-all \
  -Wl,--gc-sections \
  build/quickjs.o \
  build/quickjs-libc.o \
  build/qjs.o \
  build/dtoa.o \
  build/libunicode.o \
  build/libregexp.o \
  -o qjs-embedded-gcc \
  -lm -pthread

guest271314 Jul 21, 2026
Author

The WAT for qjs-wasi.wasm is 10.8 MiB.

guest271314 · 2026-07-19T22:13:18Z

guest271314
Jul 19, 2026
Author

Removing libregexp from the build doesn't change anything.

0 replies

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