[OpenMP][SIMD][FIX] Use conservative "omp simd ordered" lowering (#123867)

A proposed fix for #95611 [OpenMP][SIMD] ordered has no effect in a loop
SIMD region as of LLVM 18.1.0

Changes:

- Implement new lowering behavior: Conservatively serialize "omp simd"
loops that have `omp simd ordered` directive to prevent incorrect
vectorization (which results in incorrect execution behavior of the
miscompiled program).

Implementation outline:

- We start with the optimistic default initial value of
`LoopStack.setParallel(/Enable=/true);` in
`CodeGenFunction::EmitOMPSimdInit(const OMPLoopDirective &D)`.
- We only disable the loop parallel memory access assumption with `if
(HasOrderedDirective) LoopStack.setParallel(/Enable=/false);` using the
`HasOrderedDirective` (which tests for the presence of an
`OMPOrderedDirective`).
- This results in no longer incorrectly vectorizing the loop when the
`omp simd ordered` directive is present.

Motivation: We'd like to prevent incorrect vectorization of the loops
marked with the `#pragma omp ordered simd` directive which has
previously resulted in miscompiled code.

At the same time, we'd like the usage outside of the `#pragma omp
ordered simd` context to remain unaffected: Note that in the test
"clang/test/OpenMP/ordered_codegen.cpp" we only "lose" the
`!llvm.access.group` metadata in `foo_simd` alone.

This is conservative, in that it's possible some of the loops would be
possible to vectorize, but we prefer to avoid miscompilation of the
loops that are currently illegal to vectorize.

A concrete example follows:

```cpp
// "test.c"
#include <float.h>
#include <math.h>
#include <omp.h>
#include <stdio.h>
#include <stdlib.h>
#include <time.h>

int compare_float(float x1, float x2, float scalar) {
    const float diff = fabsf(x1 - x2);
    x1 = fabsf(x1);
    x2 = fabsf(x2);
    const float l = (x2 > x1) ? x2 : x1;
    if (diff <= l * scalar * FLT_EPSILON)
        return 1;
    else
        return 0;
}

#define ARRAY_SIZE 256

__attribute__((noinline)) void initialization_loop(
    float X[ARRAY_SIZE][ARRAY_SIZE], float Y[ARRAY_SIZE][ARRAY_SIZE]) {
    const float max = 1000.0;
    srand(time(NULL));
    for (int r = 0; r < ARRAY_SIZE; r++) {
        for (int c = 0; c < ARRAY_SIZE; c++) {
            X[r][c] = ((float)rand() / (float)(RAND_MAX)) * max;
            Y[r][c] = X[r][c];
        }
    }
}

__attribute__((noinline)) void omp_simd_loop(float X[ARRAY_SIZE][ARRAY_SIZE]) {
    for (int r = 1; r < ARRAY_SIZE; ++r) {
        for (int c = 1; c < ARRAY_SIZE; ++c) {
#pragma omp simd
            for (int k = 2; k < ARRAY_SIZE; ++k) {
#pragma omp ordered simd
                X[r][k] = X[r][k - 2] + sinf((float)(r / c));
            }
        }
    }
}

__attribute__((noinline)) int comparison_loop(float X[ARRAY_SIZE][ARRAY_SIZE],
                                              float Y[ARRAY_SIZE][ARRAY_SIZE]) {
    int totalErrors_simd = 0;
    const float scalar = 1.0;
    for (int r = 1; r < ARRAY_SIZE; ++r) {
        for (int c = 1; c < ARRAY_SIZE; ++c) {
            for (int k = 2; k < ARRAY_SIZE; ++k) {
                Y[r][k] = Y[r][k - 2] + sinf((float)(r / c));
            }
        }
        // check row for simd update
        for (int k = 0; k < ARRAY_SIZE; ++k) {
            if (!compare_float(X[r][k], Y[r][k], scalar)) {
                ++totalErrors_simd;
            }
        }
    }
    return totalErrors_simd;
}

int main(void) {
    float X[ARRAY_SIZE][ARRAY_SIZE];
    float Y[ARRAY_SIZE][ARRAY_SIZE];

    initialization_loop(X, Y);
    omp_simd_loop(X);
    const int totalErrors_simd = comparison_loop(X, Y);

    if (totalErrors_simd) {
        fprintf(stdout, "totalErrors_simd: %d \n", totalErrors_simd);
        fprintf(stdout, "%s : %d - FAIL: error in ordered simd computation.\n",
                __FILE__, __LINE__);
    } else {
        fprintf(stdout, "Success!\n");
    }

    return totalErrors_simd;
}
```

Before:

```
$ clang -fopenmp-simd -O3 -ffast-math -lm test.c -o test && ./test
totalErrors_simd: 15408
test.c : 76 - FAIL: error in ordered simd computation.
```

clang 19.1.0: https://godbolt.org/z/6EvhxqEhe

After:

```
$ clang -fopenmp-simd -O3 -ffast-math test.c -o test && ./test
Success!
```

Co-authored-by: Matt P. Dziubinski <[email protected]>

Author: MattPD

clang/lib/CodeGen/CGStmtOpenMP.cpp

@@ -2457,10 +2457,86 @@ static void emitSimdlenSafelenClause(CodeGenFunction &CGF,
   }
 }
 
+// Check for the presence of an `OMPOrderedDirective`,
+// i.e., `ordered` in `#pragma omp ordered simd`.
+//
+// Consider the following source code:
+// ```
+// __attribute__((noinline)) void omp_simd_loop(float X[ARRAY_SIZE][ARRAY_SIZE])
+// {
+//     for (int r = 1; r < ARRAY_SIZE; ++r) {
+//         for (int c = 1; c < ARRAY_SIZE; ++c) {
+// #pragma omp simd
+//             for (int k = 2; k < ARRAY_SIZE; ++k) {
+// #pragma omp ordered simd
+//                 X[r][k] = X[r][k - 2] + sinf((float)(r / c));
+//             }
+//         }
+//     }
+// }
+// ```
+//
+// Suppose we are in `CodeGenFunction::EmitOMPSimdInit(const OMPLoopDirective
+// &D)`. By examining `D.dump()` we have the following AST containing
+// `OMPOrderedDirective`:
+//
+// ```
+// OMPSimdDirective 0x1c32950
+// `-CapturedStmt 0x1c32028
+//   |-CapturedDecl 0x1c310e8
+//   | |-ForStmt 0x1c31e30
+//   | | |-DeclStmt 0x1c31298
+//   | | | `-VarDecl 0x1c31208  used k 'int' cinit
+//   | | |   `-IntegerLiteral 0x1c31278 'int' 2
+//   | | |-<<<NULL>>>
+//   | | |-BinaryOperator 0x1c31308 'int' '<'
+//   | | | |-ImplicitCastExpr 0x1c312f0 'int' <LValueToRValue>
+//   | | | | `-DeclRefExpr 0x1c312b0 'int' lvalue Var 0x1c31208 'k' 'int'
+//   | | | `-IntegerLiteral 0x1c312d0 'int' 256
+//   | | |-UnaryOperator 0x1c31348 'int' prefix '++'
+//   | | | `-DeclRefExpr 0x1c31328 'int' lvalue Var 0x1c31208 'k' 'int'
+//   | | `-CompoundStmt 0x1c31e18
+//   | |   `-OMPOrderedDirective 0x1c31dd8
+//   | |     |-OMPSimdClause 0x1c31380
+//   | |     `-CapturedStmt 0x1c31cd0
+// ```
+//
+// Note the presence of `OMPOrderedDirective` above:
+// It's (transitively) nested in a `CapturedStmt` representing the pragma
+// annotated compound statement. Thus, we need to consider this nesting and
+// include checking the `getCapturedStmt` in this case.
+static bool hasOrderedDirective(const Stmt *S) {
+  if (isa<OMPOrderedDirective>(S))
+    return true;
+
+  if (const auto *CS = dyn_cast<CapturedStmt>(S))
+    return hasOrderedDirective(CS->getCapturedStmt());
+
+  for (const Stmt *Child : S->children()) {
+    if (Child && hasOrderedDirective(Child))
+      return true;
+  }
+
+  return false;
+}
+
+static void applyConservativeSimdOrderedDirective(const Stmt &AssociatedStmt,
+                                                  LoopInfoStack &LoopStack) {
+  // Check for the presence of an `OMPOrderedDirective`
+  // i.e., `ordered` in `#pragma omp ordered simd`
+  bool HasOrderedDirective = hasOrderedDirective(&AssociatedStmt);
+  // If present then conservatively disable loop vectorization
+  // analogously to how `emitSimdlenSafelenClause` does.
+  if (HasOrderedDirective)
+    LoopStack.setParallel(/*Enable=*/false);
+}
+
 void CodeGenFunction::EmitOMPSimdInit(const OMPLoopDirective &D) {
   // Walk clauses and process safelen/lastprivate.
   LoopStack.setParallel(/*Enable=*/true);
   LoopStack.setVectorizeEnable();
+  const Stmt *AssociatedStmt = D.getAssociatedStmt();
+  applyConservativeSimdOrderedDirective(*AssociatedStmt, LoopStack);
   emitSimdlenSafelenClause(*this, D);
   if (const auto *C = D.getSingleClause<OMPOrderClause>())
     if (C->getKind() == OMPC_ORDER_concurrent)