[llvm] [LoopUtils] prevent negative estimated trip count result (PR #195610)

[Arda Serdar Pektezol via llvm-commits]

pektezol wrote:

> > @jdenny-ornl so, should we keep it like this and merge it?
> 
> No. I suggest treating estimated trip counts as unsigned:
> 
> ```
> diff --git a/llvm/lib/Transforms/Utils/LoopPeel.cpp b/llvm/lib/Transforms/Utils/LoopPeel.cpp
> index 4c67d5a9ae68..3ae2a7819008 100644
> --- a/llvm/lib/Transforms/Utils/LoopPeel.cpp
> +++ b/llvm/lib/Transforms/Utils/LoopPeel.cpp
> @@ -32,6 +32,7 @@
>  #include "llvm/IR/PatternMatch.h"
>  #include "llvm/IR/ProfDataUtils.h"
>  #include "llvm/Support/Casting.h"
> +#include "llvm/Support/CheckedArithmetic.h"
>  #include "llvm/Support/CommandLine.h"
>  #include "llvm/Support/Debug.h"
>  #include "llvm/Support/raw_ostream.h"
> @@ -876,7 +877,9 @@ void llvm::computePeelCount(Loop *L, unsigned LoopSize,
>      LLVM_DEBUG(dbgs() << "Profile-based estimated trip count is "
>                        << *EstimatedTripCount << "\n");
>  
> -    if (*EstimatedTripCount + AlreadyPeeled <= MaxPeelCount) {
> +    std::optional<unsigned> TotalPeeled =
> +        llvm::checkedAddUnsigned(*EstimatedTripCount, AlreadyPeeled);
> +    if (TotalPeeled && *TotalPeeled <= MaxPeelCount) {
>        unsigned PeelCount = *EstimatedTripCount;
>        LLVM_DEBUG(dbgs() << "Peeling first " << PeelCount << " iterations.\n");
>        PP.PeelCount = PeelCount;
> diff --git a/llvm/lib/Transforms/Utils/LoopUtils.cpp b/llvm/lib/Transforms/Utils/LoopUtils.cpp
> index 9c7d10629adf..b97be6e40da0 100644
> --- a/llvm/lib/Transforms/Utils/LoopUtils.cpp
> +++ b/llvm/lib/Transforms/Utils/LoopUtils.cpp
> @@ -926,7 +926,8 @@ llvm::getLoopEstimatedTripCount(Loop *L,
>    // historically assume that llvm::getLoopEstimatedTripCount always returns a
>    // positive count or std::nullopt.  Thus, return std::nullopt when
>    // llvm.loop.estimated_trip_count is 0.
> -  if (auto TC = getOptionalIntLoopAttribute(L, LLVMLoopEstimatedTripCount)) {
> +  if (std::optional<unsigned> TC =
> +          getOptionalIntLoopAttribute(L, LLVMLoopEstimatedTripCount)) {
>      LLVM_DEBUG(dbgs() << "getLoopEstimatedTripCount: "
>                        << LLVMLoopEstimatedTripCount << " metadata has trip "
>                        << "count of " << *TC
> ```

Ah yes. Updated, thank you for the review!

https://github.com/llvm/llvm-project/pull/195610