RFR: 8365312: GCC 12 cannot compile SVE on aarch64 with auto-var-init pattern [v2]

[Fei Gao]

> `-ftrivial-auto-var-init=pattern` [1] supported by gcc-12 was added recently to help detect uses of uninitialized memory. [2] Under this extension mode, `__builtin_clear_padding` is called to clear the padding bits in object representation. But the builtin function does not support variable length aggregates [3].
> 
> `vfloat2_sve_sleef` is a typedef of `svfloat32x2_t`, which is defined as an opaque sizeless type in ARM C Language Extensions (ACLE) [4]. When `__builtin_clear_padding` is applied to such a type, it triggers the unsupported code path and results in build failures when compiling libsleef on Linux-AArch64, as reported in JDK-8364185 [5].
> 
> Switching the initialization mode with gcc-12 from `pattern` to `zero` avoids the use of this unsupported `__builtin_clear_padding` and resolves the issue.
> 
> I did not encounter similar problems when building with Clang-16 [6].
> 
> This patch changes the initialization mode from `pattern` to `zero` when compiling libsleef SVE component on Linux-AArch64 with gcc-12 to fix the build failure and help avoid potential security issues related to uninitialized padding
> 
> Testing (combined with https://github.com/openjdk/jdk/pull/26577):
> 1. Release and fastdebug build with gcc-11, gcc-12, and clang-16
> 2. `test/jdk/jdk/incubator/vector` passed on both 256-bit SVE and NEON machines
> 
> [1] https://gcc.gnu.org/onlinedocs/gcc-12.1.0/gcc/Optimize-Options.html#index-ftrivial-auto-var-init
> [2] https://github.com/openjdk/jdk/commit/fae37aaae8b36fd74309b84fa1fdf017c7d932ed
> [3] https://gcc.gnu.org/pipermail/gcc-patches/2020-November/559214.html
> [4] https://arm-software.github.io/acle/main/acle.html#arm_sveh
> [5] https://bugs.openjdk.org/browse/JDK-8364185
> [6] https://releases.llvm.org/16.0.0/tools/clang/docs/ReleaseNotes.html

Fei Gao has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:

 - Add flag in debug builds and nothing in release builds
 - Merge branch 'master' into switch-pattern-to-zero
 - 8365312: __builtin_clear_padding not supported for variable length aggregates
   
   `-ftrivial-auto-var-init=pattern` [1] supported by gcc-12 was added
   recently to help detect uses of uninitialized memory. [2] Under
   this extension mode, `__builtin_clear_padding` is called to clear
   the padding bits in object representation. But the builtin function
   does not support variable length aggregates [3].
   
   `vfloat2_sve_sleef` is a typedef of `svfloat32x2_t`, which is
   defined as an opaque sizeless type in ARM C Language Extensions
   (ACLE) [4]. When `__builtin_clear_padding` is applied to such a type,
   it triggers the unsupported code path and results in build failures
   when compiling libsleef on Linux-AArch64, as reported in JDK-8364185 [5].
   
   Switching the initialization mode with gcc-12 from `pattern` to `zero`
   avoids the use of this unsupported `__builtin_clear_padding` and
   resolves the issue.
   
   I did not encounter similar problems when building with Clang-16 [6].
   
   This patch changes the initialization mode from `pattern` to `zero`
   when compiling libsleef SVE component on Linux-AArch64 with gcc-12
   to fix the build failure and help avoid potential security issues
   related to uninitialized padding
   
   Testing (combined with https://github.com/openjdk/jdk/pull/26577):
   1. Release and fastdebug build with gcc-11, gcc-12, and clang-16
   2. `test/jdk/jdk/incubator/vector` passed on both 256-bit SVE and
      NEON machines
   
   [1] https://gcc.gnu.org/onlinedocs/gcc-12.1.0/gcc/Optimize-Options.html#index-ftrivial-auto-var-init
   [2] https://github.com/openjdk/jdk/commit/fae37aaae8b36fd74309b84fa1fdf017c7d932ed
   [3] https://gcc.gnu.org/pipermail/gcc-patches/2020-November/559214.html
   [4] https://arm-software.github.io/acle/main/acle.html#arm_sveh
   [5] https://bugs.openjdk.org/browse/JDK-8364185
   [6] https://releases.llvm.org/16.0.0/tools/clang/docs/ReleaseNotes.html

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/26755/files
  - new: https://git.openjdk.org/jdk/pull/26755/files/e8bfe2b5..242ba542

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=26755&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=26755&range=00-01

  Stats: 15102 lines in 428 files changed: 8123 ins; 5212 del; 1767 mod
  Patch: https://git.openjdk.org/jdk/pull/26755.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26755/head:pull/26755

PR: https://git.openjdk.org/jdk/pull/26755