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Tighten bounds of abs() #8168

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merged 3 commits into from
Apr 5, 2024
Merged

Tighten bounds of abs() #8168

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89ee30a
Tighten bounds of abs()
rootjalex Mar 31, 2024
a282746
make abs bounds tight for non-int32 too
rootjalex Mar 31, 2024
f74dc30
make int32 min expression match non-int32 min expression
rootjalex Mar 31, 2024
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2 changes: 1 addition & 1 deletion dependencies/llvm/CMakeLists.txt
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Original file line number Diff line number Diff line change
Expand Up @@ -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)
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30 changes: 28 additions & 2 deletions src/Bounds.cpp
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Expand Up @@ -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));
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It's abs, so t is unsigned, and a.min.type() is signed. Don't you want to do the max in the unsigned type?

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If a = [2, 10] then originally you got interval.max = max(u32(-2), u32(10)) = 4294967294

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Right, I was only considering the a.min < 0 case. Doing it in the unsigned type would be: max(abs(a.min), abs(a.max)), but I'm guessing elsewhere in bounds inference isn't going to like that.

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Yeah, the original code works for a.min < 0, but I think the new code works for both negative and positive a.min. If you assume negation can't overflow, then the comparison should be done in the signed type.

} 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();
}
Expand Down Expand Up @@ -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);
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