Compilation failure if optimisations are turned off

[dylanmckay]

If there are no optimisations enabled, LLVM crashes in `LiveVariables.cpp" with the message

regalloc=... not currently supported with -O0

This occurs with

• llc (whose defaults are -O2, and so it'll only crash if you explicitly pass -O0
• avr-rust
• @4ntoine's clang fork

[dylanmckay]

Compilation succeeds if -O0 -regalloc=fast is passed. Compilation fails if any other register allocator is specified with -O0.

Compilation fails if optimisations are turned on and -regalloc=fast is turned on. Succeeds if optimisations are turned on and any other register allocator is chosen.

[4ntoine]

What's the reason for it? I believe we should make it working even if no optimizations are set (with -O0).

[dylanmckay]

What's the reason for it?

I'm not sure exactly. I spent several hours debugging it today and had no luck. Google has some interesting results which could be what we're having.

I believe we should make it working even if no optimizations are set

Me too, but I'm not sure how to fix it.

[dylanmckay]

Order of passes:

Number	Class Name	Description
0	LowerInvoke	"Lower invoke and unwind, for unwindless code generators"
1	UnreachableBlockElim	"Remove unreachable blocks from the CFG"
2	StackProtector	"Insert stack protectors"
3	VerifierLegacyPass	"Module Verifier"
4	MachineFunctionAnalysis	"Machine Function Analysis"
5	AVRDAGToDAGISel	"AVR DAG->DAG Instruction Selection"
6	AVRFrameAnalyzer	"AVR Frame Analyzer"
7	ExpandISelPseudos	"Expand ISel Pseudo-instructions"
8	LocalStackSlotPass	"Local Stack Slot Allocation"
9	AVRDynAllocaSR	"AVR dynalloca stack pointer save/restore"
10	LiveVariables	"Live Variable Analysis"
...	...	....

The problem most likely that one of the previous AVR-specific passes is messing up something it shouldn't.

[dylanmckay]

The greedy register allocator is not supposed to be used when optimizations are not enabled. Now we use whatever register allocator LLVM decides based on the optimisation level.