8364128: Improve gathering of cpu feature names using FormatBuffer #26515

ashu-mehra · 2025-07-28T18:16:16Z

This PR implements the code for cpu features names string using FormatBuffer. It also improves and extends FormatBuffer with an additional method that appends comma separated strings to the buffer. This is useful in creating the cpu features names string.
This code will also be useful in Leyden to implement cpu feature check for AOTCodeCache [0].
Platforms affected: x86-64 and aarch64
Other platforms can be done if and when Leyden changes are ported to them.

[0] openjdk/leyden#84

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Issue

JDK-8364128: Improve gathering of cpu feature names using FormatBuffer (Enhancement - P4)

Contributors

Johan Sjölen <[email protected]>

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$ git fetch https://git.openjdk.org/jdk.git pull/26515/head:pull/26515
$ git checkout pull/26515

Update a local copy of the PR:
$ git checkout pull/26515
$ git pull https://git.openjdk.org/jdk.git pull/26515/head

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$ git pr checkout 26515

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https://git.openjdk.org/jdk/pull/26515.diff

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Signed-off-by: Ashutosh Mehra <[email protected]>

bridgekeeper · 2025-07-28T18:17:01Z

👋 Welcome back asmehra! A progress list of the required criteria for merging this PR into master will be added to the body of your pull request. There are additional pull request commands available for use with this pull request.

openjdk · 2025-07-28T18:17:18Z

❗ This change is not yet ready to be integrated.
See the Progress checklist in the description for automated requirements.

openjdk · 2025-07-28T18:18:01Z

@ashu-mehra The following label will be automatically applied to this pull request:

hotspot

When this pull request is ready to be reviewed, an "RFR" email will be sent to the corresponding mailing list. If you would like to change these labels, use the /label pull request command.

mlbridge · 2025-07-28T18:21:43Z

Webrevs

jdksjolen

It seems like we can just use a stringStream instead and not have to deal with any overflow of some buffer, since we strdup at the end.

jdksjolen · 2025-07-29T06:46:17Z

src/hotspot/share/runtime/abstract_vm_version.hpp

+#define CPU_INFO_BUF_SIZE 1024
+#endif // CPU_INFO_BUF_SIZE
+
+template<size_t bufsz> class FormatBuffer;


Style: template <size_t bufsz>.

jdksjolen · 2025-07-29T06:48:59Z

src/hotspot/share/runtime/abstract_vm_version.hpp

+
+template<size_t bufsz> class FormatBuffer;
+
+using CpuInfoBuffer = FormatBuffer<CPU_INFO_BUF_SIZE>;


Is it important that we avoid dynamic allocation at this stage of VM initialization? Is that why we can't use stringStream instead?

Yeah, I think stringStream can be used as well. Dynamic allocation shouldn't be a concern. I can set the initial size to be same as the CPU_INFO_BUF_SIZE to reduce churn a bit. I will try this out. Thanks for the suggestion.

jdksjolen · 2025-07-29T06:54:32Z

src/hotspot/cpu/x86/vm_version_x86.cpp

-  _features_string = extract_features_string(_cpu_info_string,
-                                             strnlen(_cpu_info_string, sizeof(buf)),
-                                             cpu_info_size);
+  _cpu_info_string = os::strdup(info_buffer);


Sorry, I'm going to need some help here :-). How can passing a FormatBuffer to os::strdup not fail to compile?

And here we do os::strdup, so now we do have dynamic allocation, so maybe just use a stringStream?

Sorry, I'm going to need some help here :-). How can passing a FormatBuffer to os::strdup not fail to compile?

It works because of this trick in its base class FormatBufferBase:

operator const char *() const { return _buf; }

jdksjolen · 2025-07-29T06:59:46Z

src/hotspot/cpu/x86/vm_version_x86.cpp

+  assert(!info_buffer.overflow(), "not enough buffer size");
+  info_buffer.append(", ");
+  assert(!info_buffer.overflow(), "not enough buffer size");
+  int features_offset = info_buffer.length();
+  insert_features_names(_features, info_buffer);


Can't you just check the overflow at the end?

jdksjolen · 2025-07-29T07:16:58Z

src/hotspot/share/utilities/formatBuffer.hpp

+  // Appends comma separated strings obtained by mapping given range of numbers to strings
+  template<typename FN>
+  void insert_string_list(int start, int limit, FN fn) {
+    bool first = true;
+    for (int i = start; i < limit; i++) {
+      const char* str = fn(i);
+      if (str == nullptr) {
+        continue;
+      }
+      const char* comma = first ? "" : ", ";
+      int result = append("%s%s", comma, str);
+      if (result < 0) {
+        return;
+      }
+      first = false;
+    }
+    return;
+  }


This method seems overly specific to the use-case.

Something like this is more widely applicable.

// Append strings returned by gen, separating each with separator. // Stops when gen returns null or when buffer is out of space. template <typename Generator> void join(Generator gen, const char* separator) { bool first = true; const char* str = gen(); while (str != nullptr) { const char* sep = first ? "" : separator; int result = append("%s%s", sep, str); if (result < 0) { return; } first = false; } return; }

Example usage:

void VM_Version::insert_features_names(uint64_t features, CpuInfoBuffer& info_buffer) { int i = 0; info_buffer.join([&]() { while (!supports_feature((VM_Version::Feature_Flag)i) && i < MAX_CPU_FEATURES) { i++; } if (i >= MAX_CPU_FEATURES) { return nullptr; } return _features_names[i]; }, ", "); assert(!info_buffer.overflow(), "not enough buffer size"); }

+1
I can add this to the stringStream class.

Signed-off-by: Ashutosh Mehra <[email protected]>

ashu-mehra · 2025-07-30T14:08:38Z

/contributor add @jdksjolen

openjdk · 2025-07-30T14:09:22Z

@ashu-mehra
Contributor Johan Sjölen <[email protected]> successfully added.

Signed-off-by: Ashutosh Mehra <[email protected]>

ashu-mehra · 2025-07-30T16:57:31Z

@jdksjolen I updated the code as per your suggestion. Please review.

8364128: Improve gathering of cpu feature names using FormatBuffer

704b12c

Signed-off-by: Ashutosh Mehra <[email protected]>

openjdk bot added the rfr Pull request is ready for review label Jul 28, 2025

openjdk bot added the hotspot [email protected] label Jul 28, 2025

jdksjolen reviewed Jul 29, 2025

View reviewed changes

ashu-mehra added 4 commits July 29, 2025 17:20

Use stringStream instead of FormatBuffer

d08c178

Signed-off-by: Ashutosh Mehra <[email protected]>

Revert FormatBuffer changes

7e031dd

Signed-off-by: Ashutosh Mehra <[email protected]>

Add changes missed in vm_version_bsd_aarch64.cpp

6ab4699

Signed-off-by: Ashutosh Mehra <[email protected]>

Fix compile failures

f3c80d7

Signed-off-by: Ashutosh Mehra <[email protected]>

More compile failure fix

21383a7

Signed-off-by: Ashutosh Mehra <[email protected]>


		template<size_t bufsz> class FormatBuffer;

		using CpuInfoBuffer = FormatBuffer<CPU_INFO_BUF_SIZE>;

8364128: Improve gathering of cpu feature names using FormatBuffer #26515

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8364128: Improve gathering of cpu feature names using FormatBuffer #26515

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ashu-mehra commented Jul 30, 2025

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ashu-mehra commented Jul 28, 2025 •

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