emmalloc: Ask sbrk for aligned sizes (#20759)

If we give it an unaligned size, then it will align it, and then memory will
seem non-contiguous. That is, if we sbrk 4 bytes from address 0 then it
returns 0 (the start of the allocation), and logically we reserved the range
0-4. Our next sbrk of any amount will return 8, because sbrk aligns up the
address to multiples of 8. So it seems like we have a region 0-4 and another
8-. Because of this emmalloc would generate separate root regions for
them, that is, they would not get merged because of the 4 bytes of dead
space in the middle.

With this PR, the amount of new regions created in
test_emmalloc_memvalidate_verbose goes from 1311 to 1234. I'm not sure
if it's worth testing that, as it would make updating the test difficult, and this
is such an internal detail of emmalloc.

Also add some verbose logging that helps debug this.

Noticed in #20704 (comment)

Author: kripken

system/lib/emmalloc.c

@@ -486,6 +486,12 @@ static bool claim_more_memory(size_t numBytes)
   validate_memory_regions();
 #endif
 
+  // Make sure we send sbrk requests of aligned sizes. If we do not do that then
+  // it will not return contiguous regions, which leads to use creating more
+  // root regions below, inefficiently. (Note that we assume our alignment is
+  // identical to sbrk's, see the assumptions at the start of this file.)
+  numBytes = (size_t)ALIGN_UP(numBytes, MALLOC_ALIGNMENT);
+
   // Claim memory via sbrk
   uint8_t *startPtr = (uint8_t*)sbrk(numBytes);
   if ((intptr_t)startPtr == -1)
@@ -510,6 +516,9 @@ static bool claim_more_memory(size_t numBytes)
   uint8_t *previousSbrkEndAddress = listOfAllRegions ? listOfAllRegions->endPtr : 0;
   if (startPtr == previousSbrkEndAddress)
   {
+#ifdef EMMALLOC_VERBOSE
+    MAIN_THREAD_ASYNC_EM_ASM(err('claim_more_memory: expanding previous'));
+#endif
     Region *prevEndSentinel = prev_region((Region*)startPtr);
     assert(debug_region_is_consistent(prevEndSentinel));
     assert(region_is_in_use(prevEndSentinel));
@@ -536,6 +545,9 @@ static bool claim_more_memory(size_t numBytes)
   else
   {
     // Create a root region at the start of the heap block
+#ifdef EMMALLOC_VERBOSE
+    MAIN_THREAD_ASYNC_EM_ASM(err('claim_more_memory: creating new root region'));
+#endif
     create_used_region(startPtr, sizeof(Region));
 
     // Dynamic heap start region:
@@ -583,6 +595,11 @@ void emmalloc_blank_slate_from_orbit()
 static void *attempt_allocate(Region *freeRegion, size_t alignment, size_t size)
 {
   ASSERT_MALLOC_IS_ACQUIRED();
+
+#ifdef EMMALLOC_VERBOSE
+  MAIN_THREAD_ASYNC_EM_ASM(out('attempt_allocate(freeRegion=' + ptrToString($0) + ',alignment=' + Number($1) + ',size=' + Number($2) + ')'), freeRegion, alignment, size);
+#endif
+
   assert(freeRegion);
   // Look at the next potential free region to allocate into.
   // First, we should check if the free region has enough of payload bytes contained

test/code_size/embind_val_wasm.json

@@ -3,8 +3,8 @@
   "a.html.gz": 431,
   "a.js": 7498,
   "a.js.gz": 3142,
-  "a.wasm": 11525,
-  "a.wasm.gz": 5766,
-  "total": 19696,
-  "total_gz": 9339
+  "a.wasm": 11533,
+  "a.wasm.gz": 5767,
+  "total": 19704,
+  "total_gz": 9340
 }

test/code_size/hello_wasm_worker_wasm.json

@@ -3,8 +3,8 @@
   "a.html.gz": 433,
   "a.js": 667,
   "a.js.gz": 458,
-  "a.wasm": 1855,
-  "a.wasm.gz": 1049,
-  "total": 3259,
-  "total_gz": 1940
+  "a.wasm": 1863,
+  "a.wasm.gz": 1051,
+  "total": 3267,
+  "total_gz": 1942
 }

test/code_size/hello_webgl2_wasm.json

@@ -3,8 +3,8 @@
   "a.html.gz": 379,
   "a.js": 4699,
   "a.js.gz": 2419,
-  "a.wasm": 10467,
-  "a.wasm.gz": 6706,
-  "total": 15735,
-  "total_gz": 9504
+  "a.wasm": 10475,
+  "a.wasm.gz": 6710,
+  "total": 15743,
+  "total_gz": 9508
 }

test/code_size/hello_webgl2_wasm2js.json

@@ -1,10 +1,10 @@
 {
   "a.html": 567,
   "a.html.gz": 379,
-  "a.js": 17912,
-  "a.js.gz": 8067,
+  "a.js": 17921,
+  "a.js.gz": 8079,
   "a.mem": 3171,
   "a.mem.gz": 2713,
-  "total": 21650,
-  "total_gz": 11159
+  "total": 21659,
+  "total_gz": 11171
 }

test/code_size/hello_webgl_wasm.json

@@ -3,8 +3,8 @@
   "a.html.gz": 379,
   "a.js": 4185,
   "a.js.gz": 2243,
-  "a.wasm": 10467,
-  "a.wasm.gz": 6706,
-  "total": 15221,
-  "total_gz": 9328
+  "a.wasm": 10475,
+  "a.wasm.gz": 6710,
+  "total": 15229,
+  "total_gz": 9332
 }

test/code_size/hello_webgl_wasm2js.json

@@ -1,10 +1,10 @@
 {
   "a.html": 567,
   "a.html.gz": 379,
-  "a.js": 17390,
-  "a.js.gz": 7895,
+  "a.js": 17399,
+  "a.js.gz": 7910,
   "a.mem": 3171,
   "a.mem.gz": 2713,
-  "total": 21128,
-  "total_gz": 10987
+  "total": 21137,
+  "total_gz": 11002
 }