ADDITIONAL SYSTEM INFORMATION :
- Operating system: Linux
- Compiler/toolchain: GCC 9.5
- Source commit: a8784a04081a11a41fe0e3bd8ab9fba6af2d3d4d (tag: jdk-17.0.18+4, branch: master)
- Build type: fastdebug
- Coverage: enabled (built with --enable-coverage)
- Build environment: Linux, compiled from source with GCC 9.5

A DESCRIPTION OF THE PROBLEM :
C2 SuperWord assertion failure in fastdebug build: assert(out == prev || prev == nullptr) failed: no branches off of store slice (superword.cpp:1181) when running a small Java test with -Xcomp -Xbatch.

Running the attached small Java program under a fastdebug build of OpenJDK 17u with -Xcomp -Xbatch triggers an internal VM error in the C2 SuperWord optimization phase. The failure is an assertion in SuperWord::mem_slice_preds at superword.cpp:1181: “no branches off of store slice”. The test does not use any unusual constructs; it performs a simple loop writing to a byte array and uses String.indexOf and Float.floatToIntBits. The bug reproduces in my environment with a recent 17u head (tag jdk-17.0.18+4), compiled fastdebug with --enable-coverage, GCC 9.5.

The program’s logical output is deterministic and does not depend on JIT; it should print the same two lines five times. Instead, after a couple of successful iterations, the VM hits the assertion in the C2 compiler. The failure saves hs_err and a compiler replay file.

This appears to be specific to fastdebug (assertion), likely in SuperWord’s handling of a store slice within the first loop that populates a byte[] with a simple linear recurrence. Disabling SuperWord avoids the crash (see Workarounds).

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
1) Save the test as Test.java
2) Compile with the fastdebug JDK:
   jdk17u/build/linux-x86_64-server-fastdebug/jdk/bin/javac Test.java
3) Run with C2 stress options:
   jdk17u/build/linux-x86_64-server-fastdebug/jdk/bin/java -Xcomp -Xbatch Test

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
f3 = 1119432081
Test.bFld = 0
f3 = 1119432081
Test.bFld = 0
f3 = 1119432081
Test.bFld = 0
f3 = 1119432081
Test.bFld = 0
f3 = 1119432081
Test.bFld = 0
ACTUAL -
After one or two successful iterations, the VM aborts with:
Internal Error (src/hotspot/share/opto/superword.cpp:1181)
assert(out == prev || prev == nullptr) failed: no branches off of store slice
Problematic frame:
V [libjvm.so+0x...] SuperWord::mem_slice_preds(Node*, Node*, GrowableArray<Node*>&)+0x...

hs_err_pidXXXX.log and replay_pidXXXX.log are produced.

---------- BEGIN SOURCE ----------

public class Test {
    
    static class Cls {
        public static byte gbByteVar = (byte)75;
    }

    public static boolean bFld = false;
    
    public void mainTest(String[] strArr1) {
        int lvInt1 = -138;
        int lvInt2 = -138;
        float f3 = 92.589F;
        String lvStr = "xmrjueawszzseso";
        byte[] lvByte = new byte[32];

        for (int TmpVar = 0; TmpVar < 32; TmpVar++) {
            lvInt2 = lvStr.indexOf(((int)(Cls.gbByteVar)));
            lvByte[TmpVar] = ((byte) ((((byte) (TmpVar)) * 5) + 12));
        }
        if (Test.bFld) {
            for (int TmpVar = 130; TmpVar > f3; TmpVar--) {
                lvInt2 = Integer.numberOfTrailingZeros(lvInt1 - lvInt2);
            }
        }
        
        System.out.println("f3 = " + Float.floatToIntBits(f3));
        System.out.println("Test.bFld = " + (Test.bFld ? 1 : 0));
    }

    public static void main(String[] strArr) {
        Test _instance = new Test();
        for (int i = 0; i < 5; i++) {
            _instance.mainTest(strArr);
        }
    }
}


---------- END SOURCE ----------