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The Bag of Holding

A bottomless satchel once possessed by a knight-errant.

Epoch Code-Generation Update

A few minutes ago, the first 64-bit self-hosted compiler for Epoch finished the code-generation process…​ unsuccessfully.

For context, this means that the 64-bit compiler (as built by the existing 32-bit compiler) was lexed, parsed, type-checked, and turned into LLVM IR. What didn’t happen is a successful code emission, i.e. the compiler is not yet producing a working executable image.

What it does produce is about 6.8 MB of errors, or just over 121,000 lines of output. This indicates that something in the code-gen process is off. We’re generating LLVM IR but it can’t be turned into machine code because it is malformed in some way.

Inspection of the error output shows that one of the biggest offenses is bad linkage on a global variable. Epoch aspires to minimize the use of global state but it’s a useful construct while bootstrapping a compiler. Fixing this mistake is trivial and reduces the error volume to much less.

In fact, the vast bulk of the output is actually the text of the LLVM IR in pretty-printed form. This "dump" is generated to help diagnose code-gen bugs, but it’s meant for much smaller programs than the entire compiler! Culling the dumped IR shows that there are in fact only 208 errors left (after the global linkage fiasco was addressed). And all of them are the same "sort" of error…​

Terminator found in the middle of a basic block!

LLVM divides code into sections called basic blocks. A single basic block represents a linear sequence of instructions, i.e. every instruction in it is executed exactly once. The way to accomplish branching flow control is to end a basic block with a branch instruction, likely a conditional branch of some kind. Branches target other basic blocks to allow different code paths to execute based on the branch.

The dreaded error "Terminator found in the middle of a basic block!" means that the constraints have been violated. Someone tried to issue a branch instruction in the middle of a block, which ruins the idea that every instruction in the block executes exactly once.

In concrete terms, this error signals a bug in the code generation process. It means that somewhere along the line, the Epoch compiler lost track of a basic block being terminated, and continued shoving instructions into it after a branch.

Thankfully, LLVM barfs a "label" when it emits this error, and that label is sufficient to locate the offending basic block:

1>  ; <label>:41                                      ; preds = %11
1>    br label %9, !dbg !3338
1>    br label %42

Sure enough, there are two branches being attempted here. The larger context is uninteresting (it’s a nested if-statement inside a binary tree insertion routine) but the specific failure appears many times, meaning that it’s probably a small number of actual code-generation bugs to solve.

Testing, 1 2 3

As with any good software, robustness in a compiler happens only when enough bugs have been fixed while simultaneously ensuring that no new ones are introduced. The best tool I know of for doing this is automated testing. Now that a compiler bug has been identified, the objective is to replicate it in as tiny a program as possible.

This "test case" provides two things: a way to reproduce the bug on-demand so a fix can be tested, and a way to detect if the bug ever reappears. The Epoch compiler test suite is still small, but invaluable for addressing this sort of problem. I will add this particular code to the test suite and hopefully have a fix in short order.

About the author

Mike Lewis

Bellevue, WA

Guild Wars 2 Project Technical Director @arenanet and author of the Epoch programming language.