8297534: Specify the size of MEMFLAGS

[jdksjolen]

Hi!

In MallocHeader we assume that MEMFLAGS can be stored in a uint8_t, see:

class MallocHeader {

  NOT_LP64(uint32_t _alt_canary);
  const size_t _size;
  const uint32_t _mst_marker;
  const uint8_t _flags;
  const uint8_t _unused;
  uint16_t _canary;
// SNIP!
  inline MEMFLAGS flags() const { return (MEMFLAGS)_flags; }

With C++11 we can specify the underlying type of an enum class, so let's just do that. If we generate 257 memflags we'll get a compile error. I don't believe that this would lead to any change in the behavior of the code.

Testing: Compiled the code. Running tier1 testing right now.

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Issue

• JDK-8297534: Specify the size of MEMFLAGS

Reviewers

• Thomas Stuefe (@tstuefe - Reviewer) ⚠️ Review applies to 046f6abc
• Thomas Schatzl (@tschatzl - Reviewer) ⚠️ Review applies to ce9a1d24

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[tstuefe]

Good and trivial.

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8297534: Specify the size of MEMFLAGS

Reviewed-by: stuefe, tschatzl

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• fd910f7: 8297384: Add IR tests for existing idealizations of arithmetic nodes
• cfe5a37: 8297556: Parse::check_interpreter_type fails with assert "must constrain OSR typestate"
• 74d3bac: 8297417: Poly1305IntrinsicFuzzTest fails with tag mismatch exception
• 0ed8b33: 8297385: Remove duplicated null typos in javadoc
• 1fb9dad: 8296419: [REDO] JDK-8295319: pending_cards_at_gc_start doesn't include cards in thread buffers
• 2f47f83: 8297569: URLPermission constructor throws IllegalArgumentException: Invalid characters in hostname after JDK-8294378
• 87d1097: 8297530: java.lang.IllegalArgumentException: Negative length on strings concatenation
• 390e69a: 8297150: Add a @sealedGraph tag to Reference
• 8effaa8: 8223783: sun/net/www/http/HttpClient/MultiThreadTest.java sometimes detect threads+1 connections
• ab1f9ff: 8051627: Invariants about java.net.URI resolve and relativize are wrong
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[tschatzl]

Imo the _flags member of MallocHeader should then also be of type MEMFLAGS as it is now defined what size it is. That saves some casting around in related code too.
I tried the change and it at least compiled, and do not see a reason not to.

[jdksjolen]

Imo the _flags member of MallocHeader should then also be of type MEMFLAGS as it is now defined what size it is. That saves some casting around in related code too. I tried the change and it at least compiled, and do not see a reason not to.

Oh yeah, I agree.

[tstuefe]

Hmm, I can live with that, though I would have preferred the raw type for clarity. After all, enum should follow the layout of the header, not the other way around.

Can you at least add a static assert that sizeof memflags is 8 bits?

[tstuefe]

Hmm, I can live with that, though I would have preferred the raw type for clarity. After all, enum should follow the layout of the header, not the other way around.

Can you at least add a static assert that sizeof memflags is 8 bits?

[tschatzl]

Lgtm.

[jdksjolen]

Hmm, I can live with that, though I would have preferred the raw type for clarity. After all, enum should follow the layout of the header, not the other way around.

Can you at least add a static assert that sizeof memflags is 8 bits?

Absolutely. Would you like that right after the definition of MEMFLAGS then? (and not after the definition of MEMFLAGS _flags in the header)

[tstuefe]

Hmm, I can live with that, though I would have preferred the raw type for clarity. After all, enum should follow the layout of the header, not the other way around.
Can you at least add a static assert that sizeof memflags is 8 bits?

Absolutely. Would you like that right after the definition of MEMFLAGS then? (and not after the definition of MEMFLAGS _flags in the header)

I don't care much as long as it explodes when MEMFLAGS are changed :-) If you add it to the hpp file, you need to give it debug.hpp too. I leave it up to you.

[jdksjolen]

Alright, I added it to allocation.hpp with a small comment. Right now gcc gives the type error in the enum definition precedence, which gives you nicer error messages. I also ensured that the static assert gives a compiler error (by removing the enum type first), which it does.

[tstuefe]

Still good.

[jdksjolen]

/integrate

Thank you for the reviews!

[openjdk]

Going to push as commit 81eb5fb.
Since your change was applied there have been 63 commits pushed to the master branch:

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• 43d1173: 8286302: Port JEP 425 to PPC64
• d610211: 8297408: Consolidate code in runtime/ErrorHandling
• 2f83b5c: 8297640: Increase buffer size for buf (insert_features_names) in Abstract_VM_Version::insert_features_names
• 50f9043: 8297451: ProcessHandleImpl should assert privilege when modifying reaper thread
• 99d3840: 8297359: RISC-V: improve performance of floating Max Min intrinsics
• 6c05771: 8295447: NullPointerException with invalid pattern matching construct in constructor call
• 76a24c3: 8297145: Add a @sealedGraph tag to ConstantDesc
• 099b42f: 8297148: Add a @sealedGraph tag to CallSite
• 85ddd8f: 8295253: Remove kludge from v1_0/PerfDataBuffer.java
• ... and 53 more: https://git.openjdk.org/jdk/compare/42c2037429a8ee6f683bbbc99fb48c540519524c...master

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@jdksjolen Pushed as commit 81eb5fb.

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