[llvm] [InstCombine] Allow freezing multiple out-of-loop values (PR #155638)

Tue Sep 9 07:03:37 PDT 2025

c-rhodes wrote:

> > > Can you please add a test where the two start values come from the same predecessor (e.g. dummy br with both successors equal)? You need to be careful about that because the phi needs to have identical values for the same predecessor.
> > > > Perhaps I'm misunderstanding, but that's not legal IR? https://godbolt.org/z/8MYM1Yf5b
> > 
> > 
> > Apologies I think I understand your concern now. In this example: https://godbolt.org/z/W5e3rfx1E we want to avoid:
> > ```
> > define void @fold_phi_multiple_identical_start_values(i1 %c, i32 %init, i32 %n) {
> > entry:
> >   %init.fr.0 = freeze i32 %init
> >   %init.fr.1 = freeze i32 %init
> >   br i1 %c, label %loop, label %loop
> > 
> > loop:
> >   %i = phi i32 [ %init.fr.0, %entry ], [ %init.fr.1, %entry ], [ %i.next, %loop ]
> >   %i.next = add nsw nuw i32 %i, 1
> >   %cond = icmp eq i32 %i.next, %n
> >   br i1 %cond, label %loop, label %exit
> > 
> > exit:
> >   ret void
> > }
> > ```
> > 
> > 
> >     
> >       
> >     
> > 
> >       
> >     
> > 
> >     
> >   
> > the final IR is correct with `%init.fr` for both `%entry` incoming values for this.
> 
> Yes, exactly. I think it ends up being correct by accident because we later CSE the two freezes. You may see intermediate invalid IR using `-debug-counter`.

Ah yes, you're right:
```
build/bin/opt -passes=instcombine -o - fold_phi_multiple_start_values.ll -S -debug-counter=instcombine-visit=1-2
PHI node has multiple entries for the same basic block with different incoming values!
  %i = phi i32 [ %init.fr, %entry ], [ %init.fr1, %entry ], [ %i.next, %loop ]
label %entry
  %init.fr1 = freeze i32 %init
  %init.fr = freeze i32 %init
LLVM ERROR: Broken module found, compilation aborted!
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace and instructions to reproduce the bug.
Stack dump:
0.      Program arguments: build/bin/opt -passes=instcombine -o - fold_phi_multiple_start_values.ll -S -debug-counter=instcombine-visit=1-2
1.      Running pass "verify" on module "fold_phi_multiple_start_values.ll"
 #0 0x0000e06917fc5098 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/home/culrho01/llvm-project/build/bin/../lib/libLLVMSupport.so.22.0git+0x245098)
 #1 0x0000e06917fc2a28 llvm::sys::RunSignalHandlers() (/home/culrho01/llvm-project/build/bin/../lib/libLLVMSupport.so.22.0git+0x242a28)
 #2 0x0000e06917fc5ea8 SignalHandler(int, siginfo_t*, void*) Signals.cpp:0:0
 #3 0x0000e0691c325968 (linux-vdso.so.1+0x968)
 #4 0x0000e069178c7608 __pthread_kill_implementation ./nptl/pthread_kill.c:44:76
 #5 0x0000e0691787cb3c raise ./signal/../sysdeps/posix/raise.c:27:6
 #6 0x0000e06917867e00 abort ./stdlib/abort.c:81:7
 #7 0x0000e06917effab0 llvm::report_fatal_error(llvm::Twine const&, bool) (/home/culrho01/llvm-project/build/bin/../lib/libLLVMSupport.so.22.0git+0x17fab0)
 #8 0x0000e06917eff8e4 llvm::report_fatal_error(llvm::Twine const&, bool) (/home/culrho01/llvm-project/build/bin/../lib/libLLVMSupport.so.22.0git+0x17f8e4)
 #9 0x0000e0691869b8b8 llvm::VerifierPass::run(llvm::Function&, llvm::AnalysisManager<llvm::Function>&) (/home/culrho01/llvm-project/build/bin/../lib/libLLVMCore.so.22.0git+0x53b8b8)
#10 0x0000e0691865caa8 llvm::PassManager<llvm::Module, llvm::AnalysisManager<llvm::Module>>::run(llvm::Module&, llvm::AnalysisManager<llvm::Module>&) (/home/culrho01/llvm-project/build/bin/../lib/libLLVMCore.so.22.0git+0x4fcaa8)
#11 0x0000e0691c2a7128 llvm::runPassPipeline(llvm::StringRef, llvm::Module&, llvm::TargetMachine*, llvm::TargetLibraryInfoImpl*, llvm::ToolOutputFile*, llvm::ToolOutputFile*, llvm::ToolOutputFile*, llvm::StringRef, llvm::ArrayRef<llvm::PassPlugin>, llvm::ArrayRef<std::function<void (llvm::PassBuilder&)>>, llvm::opt_tool::OutputKind, llvm::opt_tool::VerifierKind, bool, bool, bool, bool, bool, bool, bool, bool) (/home/culrho01/llvm-project/build/bin/../lib/libLLVMOptDriver.so.22.0git+0x37128)
#12 0x0000e0691c2b5dd8 optMain (/home/culrho01/llvm-project/build/bin/../lib/libLLVMOptDriver.so.22.0git+0x45dd8)
#13 0x0000e069178684c4 __libc_start_call_main ./csu/../sysdeps/nptl/libc_start_call_main.h:74:3
#14 0x0000e06917868598 call_init ./csu/../csu/libc-start.c:128:20
#15 0x0000e06917868598 __libc_start_main ./csu/../csu/libc-start.c:347:5
#16 0x0000b0b3c1d10bf0 _start (build/bin/opt+0x10bf0)
[1]    1100609 abort (core dumped)  build/bin/opt -passes=instcombine -o - fold_phi_multiple_start_values.ll -S
```

I wasn't aware of that option, thanks!

> 
> > > By the way, this change isn't actually what I had in mind... I'd expect multiple starting values to get canonicalized away by loop simplify form.
> > 
> > 
> > You're right, if the multiple starting values are well-defined the freeze can be removed with a combination of loop-simplify and instcombine: https://godbolt.org/z/dh7jq5qov
> > > I think the more interesting extension here is the case where you need to freeze both the start value and the step value. So something like {start,+,step} where both values are not known non-poison.
> > 
> > 
> > Seems this is already handled: https://godbolt.org/z/9oa4YvnYz
> 
> The case I had in mind is this one: https://godbolt.org/z/cczGhW8K9 Where we'd freeze both init and step.

Ah ok thanks, will see if it can handle that then.

I have been looking at a fix for https://github.com/llvm/llvm-project/pull/157112 separately and have a small change to support non-recurrence PHIs in `pushFreezeToPreventPoisonFromPropagating` which fixes it, but I wouldn't be surprised if there's some edge case I've not considered. I'll create a PR for that.

https://github.com/llvm/llvm-project/pull/155638