[llvm] change contents of ScalarEvolution from private to protected (PR #83052)

[William Moses via llvm-commits]

================
@@ -0,0 +1,70 @@
+
+#include "llvm/Analysis/AliasAnalysis.h"
+#include "llvm/Analysis/LoopAnalysisManager.h"
+#include "llvm/Analysis/TargetLibraryInfo.h"
+
+#include "llvm/IR/Function.h"
+
+#include "llvm/IR/Instructions.h"
+#include "llvm/Transforms/Utils/ValueMapper.h"
+#include <deque>
+
+// TODO note this doesn't go through [loop, unreachable], and we could get more
+// performance by doing this can consider doing some domtree magic potentially
+static inline llvm::SmallPtrSet<llvm::BasicBlock *, 4>
+getGuaranteedUnreachable(llvm::Function *F) {
----------------
wsmoses wrote:

Ah, yeah so basically the gist is as follows.

Suppose we have some code from the loop that results in unreacahble blocks. For example:

```c
for (int i=0; i<N; i++) {
    if (i > array.size()) {
       printf("Bounds error\n");
       exit(1);
    }
    something(i);
}
```

Again in our (enzyme) case here, we can assume we only need the loop limits if we successfully run the function (e.g. exit via a return). Thus we'd prefer to learn that this look has a bound of N, rather than unknown (since otherwise the loop could exit at an arbitrary time, via an exit call / unreachable).

This actually makes a huge performance difference in practice since a lot of codes have this bounds checking (and other related things), and not deducing the fixed bound forces us to use a dynamic reallocation rather than a static allocation size of N.

https://github.com/llvm/llvm-project/pull/83052