[miscompile] Don't de-negate and change direction of shifts-by-unsigned

I'm afraid the problem is really obvious:
https://github.com/halide/Halide/blob/b5f024fa83b6f1cfe5e83a459c9378b7c5bf096d/src/CodeGen_LLVM.cpp#L2628-L2649
^ the shift direction is treated as flippable by the codegen
iff the shift amount is signed :)

The newly-added test fails without the fix.

I've hit this when writing tests for halide#6775

src/Simplify_Call.cpp

@@ -132,7 +132,7 @@ Expr Simplify::visit(const Call *op, ExprInfo *bounds) {
  // If we know the sign of this shift, change it to an unsigned shift.
  if (b_info.min_defined && b_info.min >= 0) {
  b = mutate(cast(b.type().with_code(halide_type_uint), b), nullptr);
- } else if (b_info.max_defined && b_info.max <= 0) {
+ } else if (b.type().is_int() && b_info.max_defined && b_info.max <= 0) {
  result_op = Call::get_intrinsic_name(op->is_intrinsic(Call::shift_right) ? Call::shift_left : Call::shift_right);
  b = mutate(cast(b.type().with_code(halide_type_uint), -b), nullptr);
  }
@@ -165,12 +165,14 @@ Expr Simplify::visit(const Call *op, ExprInfo *bounds) {
  }
  }
 
- // Rewrite shifts with negated RHSes as shifts of the other direction.
- if (const Sub *sub = b.as<Sub>()) {
- if (is_const_zero(sub->a)) {
- result_op = Call::get_intrinsic_name(op->is_intrinsic(Call::shift_right) ? Call::shift_left : Call::shift_right);
- b = sub->b;
- return mutate(Call::make(op->type, result_op, {a, b}, Call::PureIntrinsic), bounds);
+ // Rewrite shifts with signed negated RHSes as shifts of the other direction.
+ if (b.type().is_int()) {
+ if (const Sub *sub = b.as<Sub>()) {
+ if (is_const_zero(sub->a)) {
+ result_op = Call::get_intrinsic_name(op->is_intrinsic(Call::shift_right) ? Call::shift_left : Call::shift_right);
+ b = sub->b;
+ return mutate(Call::make(op->type, result_op, {a, b}, Call::PureIntrinsic), bounds);
+ }
  }
  }
 

test/correctness/CMakeLists.txt

@@ -285,6 +285,7 @@ tests(GROUPS correctness
  set_custom_trace.cpp
  shadowed_bound.cpp
  shared_self_references.cpp
+ shift_by_unsigned_negated.cpp
  shifted_image.cpp
  side_effects.cpp
  simd_op_check.cpp

test/correctness/shift_by_unsigned_negated.cpp

@@ -0,0 +1,46 @@
+#include "Halide.h"
+
+using namespace Halide;
+
+template<typename T>
+bool test(Func f, T f_expected, int width) {
+ Buffer<uint32_t> actual = f.realize({width});
+ for (int i = 0; i < actual.width(); i++) {
+ if (actual(i) != f_expected(i)) {
+ printf("r(%d) = %d, f_expected(%d) = %d\n",
+ i, actual(i), i, f_expected(i));
+ return false;
+ }
+ }
+ return true;
+}
+
+int main(int argc, char **argv) {
+ Buffer<uint32_t> step(31);
+ for (int i = 0; i < step.width(); i++) {
+ step(i) = -i;
+ }
+
+ bool success = true;
+ Var x;
+
+ {
+ Func f;
+ f(x) = Expr(-1U) << -step(x);
+ auto f_expected = [&](int x) {
+ return -1U << x;
+ };
+ success &= test(f, f_expected, step.width());
+ }
+ {
+ Func f;
+ f(x) = Expr(-1U) >> -step(x);
+ auto f_expected = [&](int x) {
+ return -1U >> x;
+ };
+ success &= test(f, f_expected, step.width());
+ }
+
+ if (success) printf("Success!\n");
+ return success ? 0 : -1;
+}