Tighten bounds of abs() (#8168)

* Tighten bounds of abs() * make abs bounds tight for non-int32 too * make int32 min expression match non-int32 min expression
halide · Apr 5, 2024 · a462044 · a462044
1 parent 7d99357
commit a462044
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Showing 2 changed files with 29 additions and 3 deletions.
diff --git a/dependencies/llvm/CMakeLists.txt b/dependencies/llvm/CMakeLists.txt
@@ -21,7 +21,7 @@ message(STATUS "Using LLVMConfig.cmake in: ${LLVM_DIR}")
 message(STATUS "Using ClangConfig.cmake in: ${Clang_DIR}")
 
 if (LLVM_PACKAGE_VERSION VERSION_LESS 16.0)
-    message(FATAL_ERROR "LLVM version must be 15.0 or newer")
+    message(FATAL_ERROR "LLVM version must be 16.0 or newer")
 endif ()
 
 if (LLVM_PACKAGE_VERSION VERSION_GREATER 19.0)

diff --git a/src/Bounds.cpp b/src/Bounds.cpp
@@ -1237,18 +1237,29 @@ class Bounds : public IRVisitor {
 
         if (op->is_intrinsic(Call::abs)) {
             Interval a = arg_bounds.get(0);
-            interval.min = make_zero(t);
+
             if (a.is_bounded()) {
                 if (equal(a.min, a.max)) {
                     interval = Interval::single_point(Call::make(t, Call::abs, {a.max}, Call::PureIntrinsic));
                 } else if (op->args[0].type().is_int() && op->args[0].type().bits() >= 32) {
-                    interval.max = Max::make(Cast::make(t, -a.min), Cast::make(t, a.max));
+                    interval.min = Cast::make(t, Max::make(a.min, -Min::make(make_zero(a.min.type()), a.max)));
+                    interval.max = Cast::make(t, Max::make(-a.min, a.max));
                 } else {
+                    interval.min = Cast::make(t, Max::make(a.min, -Min::make(make_zero(a.min.type()), a.max)));
                     a.min = Call::make(t, Call::abs, {a.min}, Call::PureIntrinsic);
                     a.max = Call::make(t, Call::abs, {a.max}, Call::PureIntrinsic);
                     interval.max = Max::make(a.min, a.max);
                 }
             } else {
+                if (a.has_lower_bound()) {
+                    // If a is strictly positive, then abs(a) is strictly positive.
+                    interval.min = Cast::make(t, Max::make(make_zero(a.min.type()), a.min));
+                } else if (a.has_upper_bound()) {
+                    // If a is strictly negative, then abs(a) is strictly positive.
+                    interval.min = Cast::make(t, -Min::make(make_zero(a.max.type()), a.max));
+                } else {
+                    interval.min = make_zero(t);
+                }
                 // If the argument is unbounded on one side, then the max is unbounded.
                 interval.max = Interval::pos_inf();
             }
@@ -3651,6 +3662,21 @@ void bounds_test() {
     check(scope, cast<float>(x), 0.0f, 10.0f);
 
     check(scope, cast<int32_t>(abs(cast<float>(x))), 0, 10);
+    check(scope, abs(2 + x), u32(2), u32(12));
+    check(scope, abs(x - 11), u32(1), u32(11));
+    check(scope, abs(x - 5), u32(0), u32(5));
+    check(scope, abs(2 + cast<float>(x)), 2.f, 12.f);
+    check(scope, abs(cast<float>(x) - 11), 1.f, 11.f);
+    check(scope, abs(cast<float>(x) - 5), 0.f, 5.f);
+    check(scope, abs(2 + cast<int8_t>(x)), u8(2), u8(12));
+    check(scope, abs(cast<int8_t>(x) - 11), u8(1), u8(11));
+    check(scope, abs(cast<int8_t>(x) - 5), u8(0), u8(5));
+    scope.push("x", Interval(123, Interval::pos_inf()));
+    check(scope, abs(x), u32(123), Interval::pos_inf());
+    scope.pop("x");
+    scope.push("x", Interval(Interval::neg_inf(), -123));
+    check(scope, abs(x), u32(123), Interval::pos_inf());
+    scope.pop("x");
 
     // Check some vectors
     check(scope, Ramp::make(x * 2, 5, 5), 0, 40);