RFR: 8359235: C1 compilation fails with "assert(is_single_stack() && !is_virtual()) failed: type check"

[Guanqiang Han]

On Fri, 25 Jul 2025 14:19:13 GMT, Damon Fenacci <dfenacci at openjdk.org> wrote:

>> I'm able to consistently reproduce the problem using the following command line and test program ：
>> 
>> java -Xcomp -XX:TieredStopAtLevel=1 -XX:C1MaxInlineSize=200 Test.java
>> 
>> import java.util.Arrays;
>> public class Test{
>>       public static void main(String[] args) {
>>         System.out.println("begin"); 
>>         byte[] arr1 = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
>>         byte[] arr2 = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
>>         System.out.println(Arrays.equals(arr1, arr2)); 
>>         System.out.println("end");
>>     }
>> }
>> 
>> From my analysis, the root cause appears to be a mismatch in operand handling between T_ADDRESS and T_LONG in LIR_Assembler::stack2reg, especially when the source is marked as double stack (e.g., T_LONG) and the destination as single CPU register (e.g., T_ADDRESS), leading to assertion failures like assert(is_single_stack())(because T_LONG is double_size).
>> 
>> In the test program above , the call chain is: Arrays.equals → ArraysSupport.vectorizedMismatch → LIRGenerator::do_vectorizedMismatch
>> Within the do_vectorizedMismatch() method, a move instruction constructs an LIR_Op1. During LIR to machine code generation, LIR_Assembler::stack2reg was called.
>> 
>> In this case, the src operand has type T_LONG and the dst operand has type T_ADDRESS. This combination triggers an assert in stack2reg, due to a mismatch between the stack slot type and register type handling.
>> 
>> Importantly, this path ( LIR_Assembler::stack2reg was called )  is only taken when src is forced onto the stack. To reliably trigger this condition, the test is run with the -Xcomp option to force compilation and increase register pressure. 
>> 
>> A reference to the relevant code paths is provided below :
>> <img width="1260" height="720" alt="image1" src="https://github.com/user-attachments/assets/d0d6a8e2-4316-4475-86a6-58f5f274682c" />
>> <img width="598" height="206" alt="image2" src="https://github.com/user-attachments/assets/90d1bcdd-c9fa-4598-b8a6-101101caad9c" />
>> 
>> On 64-bit platforms, although T_ADDRESS is classified as single_size, it is in fact 64 bits wide ,represent a single 64-bit general-purpose register  and  it can hold a T_LONG value, which is also 64 bits.
>> 
>> However, T_LONG is defined as double_size, requiring two local variable slots or a pair of registers in the JVM's abstract model. This mismatch stems from the fact that T_ADDRESS is platform-dependent: it's 32 bits on 32-bit platforms, and 64 bits on 64-bit platforms — yet its size class...
>
> src/hotspot/share/c1/c1_LIR.hpp line 430:
> 
>> 428:   int single_stack_ix() const  { assert(is_single_stack() && !is_virtual(), "type check"); return (int)data(); }
>> 429:   int double_stack_ix() const  { assert(is_double_stack() && !is_virtual(), "type check"); return (int)data(); }
>> 430:   int stack_ix() const  { assert((is_double_stack() || is_single_stack()) && !is_virtual(), "type check"); return (int)data(); }
> 
> Minor thing, but I would follow the alignment of the other methods.

@dafedafe Thanks for the feedback. I'll be more careful with these details next time.

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PR Review Comment: https://git.openjdk.org/jdk/pull/26462#discussion_r2233894328