Binary size skill (pytorch#17988)

### Summary
Add binary size skill and update ci with new binary size thresholds

### Test plan
ci

---------

Co-authored-by: Github Executorch <github_executorch@arm.com>

Author: lucylq

.claude/skills/binary-size/SKILL.md

@@ -0,0 +1,72 @@
+---
+name: binary-size
+description: Analyze and reduce ExecuTorch binary size. Use when investigating binary size, running size tests, or optimizing the runtime for size-constrained deployments.
+---
+
+# Binary Size
+
+## Start from the `main` branch of executorch
+Ask the user where the executorch repo is.
+
+```bash
+git checkout main && git pull
+```
+
+## Build and measure baseline
+```bash
+conda activate executorch
+bash test/build_size_test.sh
+strip -o /tmp/size_test_stripped cmake-out/test/size_test
+strip -o /tmp/size_test_all_ops_stripped cmake-out/test/size_test_all_ops
+ls -la /tmp/size_test_stripped /tmp/size_test_all_ops_stripped
+```
+
+Produces two binaries:
+- `cmake-out/test/size_test` — ExecuTorch runtime without operator implementations
+- `cmake-out/test/size_test_all_ops` — ExecuTorch runtime with portable ops
+
+## Analyze with bloaty
+```bash
+bloaty cmake-out/test/size_test -d symbols -n 30   # by symbol
+bloaty cmake-out/test/size_test -d sections        # by ELF section
+bloaty <after> -- <before>                          # diff two builds
+nm -S <binary> | sort -k2 -rn | head -30           # symbol sizes
+strings <binary> | less                             # string literals in .rodata
+```
+
+Note: `bloaty -d compileunits` requires debug info (`-g`). The Release build does not include it.
+
+## Key build flags
+Set by `test/build_size_test.sh`:
+- `CMAKE_BUILD_TYPE=Release`
+- `EXECUTORCH_OPTIMIZE_SIZE=ON` — enables `-Os`, `-fno-exceptions`, `-fno-rtti`, unwind table suppression
+- `CXXFLAGS="-fno-exceptions -fno-rtti -Wall -Werror"`
+
+## Constraints
+- Use **CMake** to build (not Buck)
+- **C++17 minimum** language standard
+- Must build on **GCC 9** (CI uses `executorch-ubuntu-22.04-gcc9-nopytorch`) and **Clang 12** — avoid compiler-specific flags or pragmas without version guards
+- Do not regress existing functionality — run tests for modified files
+- Do not change build flags in `build_size_test.sh` for size reductions
+- Do not increase latency in the core runtime
+
+## Where to look for size reductions
+- `.text`: look for large functions, template bloat, duplicate instantiations
+- `.rodata`: verbose error messages, format strings, embedded file paths (`__FILE__`)
+- `.eh_frame`: should already be suppressed when `EXECUTORCH_OPTIMIZE_SIZE=ON`
+- Static init functions (`nm -S <binary> | grep GLOBAL__sub_I`): use `constexpr` constructors to constant-initialize static arrays
+- Logging strings: `ET_LOG_ENABLED=0` in Release eliminates format strings; ensure it propagates to consumers via `PUBLIC` compile definitions on cmake targets
+- Inline header functions: watch for compile-define mismatches between library and consumer TUs (e.g. `ET_LOG_ENABLED` set in library but not in consumer)
+
+## For each change
+1. Create a branch: `git checkout -b binary-size-<N>`
+2. Implement, rebuild, measure stripped sizes
+3. Create a separate PR — one logical change per PR
+4. Record results in `binary-size-<N>.md`:
+
+| Binary | This change (N vs N-1) | Cumulative (N vs main) |
+|---|---|---|
+| `size_test` (stripped) | -X | -Y |
+| `size_test_all_ops` (stripped) | -X | -Y |
+
+5. Update the CI size threshold in `.github/workflows/pull.yml` if sizes decrease

.github/workflows/pull.yml

@@ -475,10 +475,8 @@ jobs:
         output=$(ls -la cmake-out/test/size_test)
         arr=($output)
         size=${arr[4]}
-        # threshold=48120 on devserver with gcc9
-        # todo(lfq): update once binary size is below 50kb.
-        # Note: using gcc9-nopytorch container with pinned nightly PyTorch
-        threshold="63785"
+        # Current CI size: 48008 (gcc9-nopytorch, 2026-03-06)
+        threshold="48500"
         if [[ "$size" -le "$threshold" ]]; then
           echo "Success $size <= $threshold"
         else
@@ -513,7 +511,8 @@ jobs:
         output=$(ls -la cmake-out/test/size_test)
         arr=($output)
         size=${arr[4]}
-        threshold="51752"
+        # Current CI size: 44160 (clang12, 2026-03-06)
+        threshold="45000"
         if [[ "$size" -le "$threshold" ]]; then
           echo "Success $size <= $threshold"
         else

CLAUDE.md

@@ -6,6 +6,7 @@
 - `/building` - Build runners or C++ libs
 - `/profile` - Profile execution
 - `/cortex-m` - Build, test, or develop the Cortex-M backend
+- `/binary-size` - Analyze and reduce binary size
 
 Reference docs in `.claude/`: backends, runtime-api, quantization, llm-export, faq, tokenizers