Add support for metadata.code.branch_hint section #4460

Lukasdoe · 2025-07-07T14:35:59Z

Since we're currently in the progress of upstreaming the generation of the code metadata branch hint section in llvm (PR), I also implemented support for this feature in wamr.

One important consideration for the implementation was to keep the encoding of hints generic, so that more compilation hints could be added in the future.

Lukasdoe · 2025-07-07T14:40:11Z

(rebased on main)

yamt · 2025-07-08T05:20:40Z

core/iwasm/compilation/aot_emit_control.c

+    }
+    if (hint != NULL) {
+        LLVMMetadataRef header = LLVMMDStringInContext2(
+            ctxt, "branch_weights", 14 /* strlen("branch_hint") */);


stale comment?

yamt · 2025-07-08T05:23:39Z

core/iwasm/compilation/aot_emit_control.c

+    do {                                                                     \
+        if (instr = LLVMBuildCondBr(comp_ctx->builder, value_if, block_then, \
+                                    block_else),                             \
+            !instr) {                                                        \


i think we usually use a style like:

if (!(instr = ...)) {

or

instr = ....; if (!instr) {

yamt · 2025-07-08T05:26:00Z

core/iwasm/aot/aot_runtime.h

@@ -57,6 +58,7 @@ typedef enum AOTCustomSectionType {
    AOT_CUSTOM_SECTION_ACCESS_CONTROL = 2,
    AOT_CUSTOM_SECTION_NAME = 3,
    AOT_CUSTOM_SECTION_STRING_LITERAL = 4,
+    AOT_CUSTOM_SECTION_CODE_METADATA = 5,


what is this for?

yamt · 2025-07-08T05:26:34Z

core/iwasm/aot/aot_runtime.h

@@ -35,6 +35,7 @@ extern "C" {
 * and not at the beginning of each function call */
 #define WASM_FEATURE_FRAME_PER_FUNCTION (1 << 12)
 #define WASM_FEATURE_FRAME_NO_FUNC_IDX (1 << 13)
+#define WASM_FEATURE_BRANCH_HINTS (1 << 14)


do we need this flag?
branch hinting doesn't produce any incompatibility in the aot file, does it?

yamt · 2025-07-08T05:30:03Z

build-scripts/config_common.cmake

@@ -215,6 +215,10 @@ if (NOT DEFINED WAMR_BUILD_EXTENDED_CONST_EXPR)
  set (WAMR_BUILD_EXTENDED_CONST_EXPR 0)
 endif ()

+if (NOT DEFINED WAMR_BUILD_BRANCH_HINTS)
+  set (WAMR_BUILD_BRANCH_HINTS 1)


is there any benefit to enable this besides wamr-compiler?

lum1n0us

Is there any possibility that the interpreter can also benefit from this feature?

lum1n0us · 2025-07-09T02:09:43Z

build-scripts/config_common.cmake

@@ -708,8 +718,8 @@ message (
 "       \"Tail call\" via WAMR_BUILD_TAIL_CALL: ${WAMR_BUILD_TAIL_CALL}\n"
 "       \"Threads\" via WAMR_BUILD_SHARED_MEMORY: ${WAMR_BUILD_SHARED_MEMORY}\n"
 "       \"Typed Function References\" via WAMR_BUILD_GC: ${WAMR_BUILD_GC}\n"
+"       \"Branch Hinting\" via WAMR_BUILD_BRANCH_HINTS: ${WAMR_BUILD_BRANCH_HINTS}\n"


Please maintain alphabetical order.

lum1n0us · 2025-07-09T02:17:51Z

core/config.h

@@ -579,6 +579,10 @@ unless used elsewhere */
 #define WASM_ENABLE_REF_TYPES 0
 #endif

+#ifndef WASM_ENABLE_BRANCH_HINTS
+#define WASM_ENABLE_BRANCH_HINTS 0


consistent with the content of config_common.cmake.

Suggested change

#define WASM_ENABLE_BRANCH_HINTS 0

#define WASM_ENABLE_BRANCH_HINTS 1

lum1n0us · 2025-07-09T02:18:47Z

build-scripts/config_common.cmake

+  add_definitions (-DWASM_ENABLE_BRANCH_HINTS=1)
+  message ("     branch hints enabled")
+else ()
+  message ("     branch hints disabled")


Suggested change

message (" branch hints disabled")

add_definitions (-DWASM_ENABLE_BRANCH_HINTS=0)

message (" branch hints disabled")

Make sure WASM_ENABLE_BRANCH_HINTS is defined.

lum1n0us · 2025-07-09T03:05:52Z

core/iwasm/compilation/aot.h

@@ -217,6 +218,9 @@ typedef struct AOTFunc {
    /* offset of each local, including function parameters
       and local variables */
    uint16 *local_offsets;
+#if WASM_ENABLE_BRANCH_HINTS != 0
+    uint8 *code_body_begin;
+#endif


suggest using existed filed code at line 217 instead of creating a new field.

lum1n0us · 2025-07-09T03:07:19Z

core/iwasm/compilation/aot.h

@@ -207,6 +207,7 @@ typedef struct AOTImportFunc {
 typedef struct AOTFunc {
    AOTFuncType *func_type;
    uint32 func_type_index;
+    uint32 func_index;


It seems unused.

lum1n0us · 2025-07-09T05:08:05Z

core/iwasm/interpreter/wasm.h

+    enum WASMCompilationHintType type;
+};
+struct WASMCompilationHintBranchHint {
+    void *next;


Suggested change

void *next;

struct WASMCompilationHintBranchHint *next;

lum1n0us · 2025-07-09T05:08:41Z

core/iwasm/interpreter/wasm.h

+};
+struct WASMCompilationHint {
+    struct WASMCompilationHint *next;
+    enum WASMCompilationHintType type;


need a new field like struct WASMCompilationHintBranchHint *branch_hints?

lum1n0us · 2025-07-09T05:09:39Z

core/iwasm/interpreter/wasm_loader.c

+                           uint32 error_buf_size)
+{
+    if (module->function_hints == NULL) {
+        module->function_hints = loader_malloc(


somewhere should release allocations

lum1n0us · 2025-07-09T05:09:50Z

core/iwasm/interpreter/wasm_loader.c

+        uint32 num_hints;
+        read_leb_uint32(buf, buf_end, num_hints);
+        for (uint32 j = 0; j < num_hints; ++j) {
+            struct WASMCompilationHintBranchHint *new_hint =


somewhere should release allocations

lum1n0us · 2025-07-09T05:11:29Z

core/iwasm/interpreter/wasm_loader.c

+            struct WASMCompilationHintBranchHint *new_hint =
+                loader_malloc(sizeof(struct WASMCompilationHintBranchHint),
+                              error_buf, error_buf_size);
+            new_hint->next = NULL;


since L5555 has already learned num_hints, it is better to malloc() once outside of the loop to reduce memory fragments

Lukasdoe requested review from loganek, lum1n0us, no1wudi, TianlongLiang, wenyongh, xujuntwt95329 and yamt as code owners July 7, 2025 14:36

Lukasdoe force-pushed the branch-hints branch from ac2d3b4 to 2f2575d Compare July 7, 2025 14:39

Add support for metadata.code.branch_hint section

9ea7ff6

Lukasdoe force-pushed the branch-hints branch from 2f2575d to 9ea7ff6 Compare July 7, 2025 15:15

yamt reviewed Jul 8, 2025

View reviewed changes

lum1n0us reviewed Jul 9, 2025

View reviewed changes

	#define WASM_ENABLE_BRANCH_HINTS 0
	#define WASM_ENABLE_BRANCH_HINTS 1

	message (" branch hints disabled")
	add_definitions (-DWASM_ENABLE_BRANCH_HINTS=0)
	message (" branch hints disabled")

Add support for metadata.code.branch_hint section #4460

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Add support for metadata.code.branch_hint section #4460

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Lukasdoe commented Jul 7, 2025

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yamt Jul 8, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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yamt Jul 8, 2025 •

edited

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