$ cat Test.java
class Test {

    public static String concatStringConstU(String a) {
        return new StringBuilder().append(stringSmallU).append(a).append(stringU).toString();
    }

    static final String stringU = "\u0f21\u0f22\u0f23\u0f24\u0f25\u0f26\u0f27\u0f28";
    static final String stringSmallU = "\u0f21\u0f22\u0f23";

    public static void main(String args[]){
        String a = "ABC";
        Test t = new Test();
        for (int i = 0; i < 100_000; i++){
            t.concatStringConstU(a);
        }
    }

}

Disable StrInflatedCopy match rule support for x86 with following trivial change and build a slowdebug jdk on x86_64 Linux:

diff --git a/src/hotspot/cpu/x86/x86.ad b/src/hotspot/cpu/x86/x86.ad
index 7cf669f0e27..84d3e38125b 100644
--- a/src/hotspot/cpu/x86/x86.ad
+++ b/src/hotspot/cpu/x86/x86.ad
@@ -1461,6 +1461,8 @@ const bool Matcher::match_rule_supported(int opcode) {
         return false;
       }
       break;
+ case Op_StrInflatedCopy:
+ return false;
     case Op_OnSpinWait:
       if (VM_Version::supports_on_spin_wait() == false) {
         return false;


$ ~/github/jdk-upstream/build/linux-x86_64-server-slowdebug/jdk/bin/java -XX:-BackgroundCompilation -XX:CICompilerCount=2 Test

1. First JVM crash log
  1 #
  2 # A fatal error has been detected by the Java Runtime Environment:
  3 #
  4 # Internal Error (/home/yangfei/github/jdk-upstream/src/hotspot/share/opto/graphKit.cpp:4215), pid=21009, tid=21104
  5 # assert(Matcher::match_rule_supported(Op_StrInflatedCopy)) failed: Intrinsic not supported
  6 #
  7 # JRE version: OpenJDK Runtime Environment (17.0) (slowdebug build 17-internal+0-adhoc.yangfei.jdk-upstream)
  8 # Java VM: OpenJDK 64-Bit Server VM (slowdebug 17-internal+0-adhoc.yangfei.jdk-upstream, mixed mode, tiered, compressed oops, compressed class ptrs, g1 gc, linux-amd64)
  9 # Problematic frame:
 10 # V [libjvm.so+0xa6c41c] GraphKit::inflate_string(Node*, Node*, TypeAryPtr const*, Node*)+0x3a
 11 #
 12 # Core dump will be written. Default location: Core dumps may be processed with "/usr/share/apport/apport %p %s %c %d %P %E" (or dumping to /h ome/yangfei/c2/core.21009)
 13 #
 14 # If you would like to submit a bug report, please visit:
 15 # https://bugreport.java.com/bugreport/crash.jsp
 16 #
 17
 18 --------------- S U M M A R Y ------------
 19
 20 Command Line: -XX:-BackgroundCompilation -XX:CICompilerCount=2 Test
 21
 22 Host: ubuntu18, Intel(R) Xeon(R) CPU E5-2690 v3 @ 2.60GHz, 48 cores, 377G, Ubuntu 18.04.2 LTS
 23 Time: Wed Mar 24 09:57:06 2021 CST elapsed time: 6.732316 seconds (0d 0h 0m 6s)
 24
 25 --------------- T H R E A D ---------------
 26
 27 Current thread (0x00007f7e1045efa0): JavaThread "C2 CompilerThread0" daemon [_thread_in_native, id=21104, stack(0x00007f7da9160000,0x00007f7d a9261000)]
 28
 29
 30 Current CompileTask:
 31 C2: 6732 417 b 4 java.lang.AbstractStringBuilder::append (45 bytes)
 32
 33 Stack: [0x00007f7da9160000,0x00007f7da9261000], sp=0x00007f7da925bc60, free space=1007k
 34 Native frames: (J=compiled Java code, A=aot compiled Java code, j=interpreted, Vv=VM code, C=native code)
 35 V [libjvm.so+0xa6c41c] GraphKit::inflate_string(Node*, Node*, TypeAryPtr const*, Node*)+0x3a
 36 V [libjvm.so+0xdb1000] LibraryCallKit::inline_string_copy(bool)+0x552
 37 V [libjvm.so+0xdac345] LibraryCallKit::try_to_inline(int)+0x4c3
 38 V [libjvm.so+0xdab236] LibraryIntrinsic::generate(JVMState*)+0x188
 39 V [libjvm.so+0x9026d8] Parse::do_call()+0xb80
 40 V [libjvm.so+0x10070c3] Parse::do_one_bytecode()+0x45e1
 41 V [libjvm.so+0xff5068] Parse::do_one_block()+0x4dc
 42 V [libjvm.so+0xff146f] Parse::do_all_blocks()+0x3bd
 43 V [libjvm.so+0xff0fb3] Parse::Parse(JVMState*, ciMethod*, float)+0x100b
 44 V [libjvm.so+0x6c75d4] ParseGenerator::generate(JVMState*)+0x106
 ......

Proposed patch fixed the first crash by this change:

diff --git a/src/hotspot/share/opto/c2compiler.cpp b/src/hotspot/share/opto/c2compiler.cpp
index 89b0270ab72..f2b9e3ecd6d 100644
--- a/src/hotspot/share/opto/c2compiler.cpp
+++ b/src/hotspot/share/opto/c2compiler.cpp
@@ -179,11 +179,11 @@ bool C2Compiler::is_intrinsic_supported(const methodHandle& method, bool is_virt
   switch (id) {
   case vmIntrinsics::_compressStringC:
   case vmIntrinsics::_compressStringB:
- if (!Matcher::has_match_rule(Op_StrCompressedCopy)) return false;
+ if (!Matcher::match_rule_supported(Op_StrCompressedCopy)) return false;
     break;
   case vmIntrinsics::_inflateStringC:
   case vmIntrinsics::_inflateStringB:
- if (!Matcher::has_match_rule(Op_StrInflatedCopy)) return false;
+ if (!Matcher::match_rule_supported(Op_StrInflatedCopy)) return false;
     break;
   case vmIntrinsics::_compareToL:
   case vmIntrinsics::_compareToU:

2. Second jvm crash log
  1 #
  2 # A fatal error has been detected by the Java Runtime Environment:
  3 #
  4 # Internal Error (/home/yangfei/github/jdk-upstream/src/hotspot/share/opto/loopnode.cpp:3790), pid=16853, tid=16868
  5 # assert(_ltree_root->_child == __null || C->has_loops() || only_has_infinite_loops() || C->has_exception_backedge()) failed: parsing found no loops but there are some
  6 #
  7 # JRE version: OpenJDK Runtime Environment (17.0) (slowdebug build 17-internal+0-adhoc.yangfei.jdk-upstream)
  8 # Java VM: OpenJDK 64-Bit Server VM (slowdebug 17-internal+0-adhoc.yangfei.jdk-upstream, mixed mode, tiered, compressed oops, compressed class ptrs, g1 gc, linux-amd64)
  9 # Problematic frame:
 10 # V [libjvm.so+0xe14d0e] PhaseIdealLoop::build_and_optimize(LoopOptsMode)+0x334
 11 #
 12 # Core dump will be written. Default location: Core dumps may be processed with "/usr/share/apport/apport %p %s %c %d %P %E" (or dumping to /h ome/yangfei/c2/core.16853)
 13 #
 14 # If you would like to submit a bug report, please visit:
 15 # https://bugreport.java.com/bugreport/crash.jsp
 16 #
 17
 18 --------------- S U M M A R Y ------------
 19
 20 Command Line: -XX:-BackgroundCompilation -XX:CICompilerCount=2 Test
 21
 22 Host: ubuntu18, Intel(R) Xeon(R) CPU E5-2690 v3 @ 2.60GHz, 48 cores, 377G, Ubuntu 18.04.2 LTS
 23 Time: Wed Mar 24 10:02:49 2021 CST elapsed time: 4.423882 seconds (0d 0h 0m 4s)
 24
 25 --------------- T H R E A D ---------------
 26
 27 Current thread (0x00007f39f445efa0): JavaThread "C2 CompilerThread0" daemon [_thread_in_native, id=16868, stack(0x00007f398ab5d000,0x00007f39 8ac5e000)]
 28
 29
 30 Current CompileTask:
 31 C2: 4423 421 b 4 Test::concatStringConstU (25 bytes)
 32
 33 Stack: [0x00007f398ab5d000,0x00007f398ac5e000], sp=0x00007f398ac58120, free space=1004k
 34 Native frames: (J=compiled Java code, A=aot compiled Java code, j=interpreted, Vv=VM code, C=native code)
 35 V [libjvm.so+0xe14d0e] PhaseIdealLoop::build_and_optimize(LoopOptsMode)+0x334
 36 V [libjvm.so+0x7e9c96] PhaseIdealLoop::PhaseIdealLoop(PhaseIterGVN&, LoopOptsMode)+0x10e
 37 V [libjvm.so+0x7e9eae] PhaseIdealLoop::optimize(PhaseIterGVN&, LoopOptsMode)+0x46
 38 V [libjvm.so+0x7dcd79] Compile::optimize_loops(PhaseIterGVN&, LoopOptsMode)+0xf7
 39 V [libjvm.so+0x7ddc8c] Compile::Optimize()+0xd6e
 40 V [libjvm.so+0x7d6bd6] Compile::Compile(ciEnv*, ciMethod*, int, bool, bool, bool, bool, DirectiveSet*)+0x13e4
 41 V [libjvm.so+0x6c563e] C2Compiler::compile_method(ciEnv*, ciMethod*, int, bool, DirectiveSet*)+0x128
 42 V [libjvm.so+0x7f3ac3] CompileBroker::invoke_compiler_on_method(CompileTask*)+0x889
 43 V [libjvm.so+0x7f26e7] CompileBroker::compiler_thread_loop()+0x3dd
 44 V [libjvm.so+0x815377] CompilerThread::thread_entry(JavaThread*, Thread*)+0x69
 45 V [libjvm.so+0x1273a9b] JavaThread::thread_main_inner()+0x14f
 46 V [libjvm.so+0x1273944] JavaThread::run()+0x11e
 47 V [libjvm.so+0x1270f72] Thread::call_run()+0x180
 48 V [libjvm.so+0xfbda2e] thread_native_entry(Thread*)+0x1e4

Proposed patch fixed the second crash by this change:

diff --git a/src/hotspot/share/opto/graphKit.cpp b/src/hotspot/share/opto/graphKit.cpp
index 69e3c4fa9dd..22078bdbff4 100644
--- a/src/hotspot/share/opto/graphKit.cpp
+++ b/src/hotspot/share/opto/graphKit.cpp
@@ -4228,6 +4228,8 @@ void GraphKit::inflate_string_slow(Node* src, Node* dst, Node* start, Node* coun
    * }
    */
   add_empty_predicates();
+ C->set_has_loops(true);
+
   RegionNode* head = new RegionNode(3);
   head->init_req(1, control());
   gvn().set_type(head, Type::CONTROL);