Sema: elide integer comparisons with guaranteed outcomes

1 files changed, 126 insertions, 0 deletions

diff --git a/src/Sema.zig b/src/Sema.zig
index 8c7c8b0dd7..8f00cbb425 100644
--- a/src/Sema.zig
+++ b/src/Sema.zig
@@ -28460,6 +28460,17 @@ fn cmpNumeric(
     const runtime_src: LazySrcLoc = src: {
         if (try sema.resolveMaybeUndefVal(lhs)) |lhs_val| {
             if (try sema.resolveMaybeUndefVal(rhs)) |rhs_val| {
+                // Compare ints: const vs. undefined (or vice versa)
+                if (!lhs_val.isUndef() and (lhs_ty.isInt() or lhs_ty_tag == .ComptimeInt) and rhs_ty.isInt() and rhs_val.isUndef()) {
+                    if (sema.compareIntsOnlyPossibleResult(target, lhs_val, op, rhs_ty)) |res| {
+                        return if (res) Air.Inst.Ref.bool_true else Air.Inst.Ref.bool_false;
+                    }
+                } else if (!rhs_val.isUndef() and (rhs_ty.isInt() or rhs_ty_tag == .ComptimeInt) and lhs_ty.isInt() and lhs_val.isUndef()) {
+                    if (sema.compareIntsOnlyPossibleResult(target, rhs_val, op.reverse(), lhs_ty)) |res| {
+                        return if (res) Air.Inst.Ref.bool_true else Air.Inst.Ref.bool_false;
+                    }
+                }
+
                 if (lhs_val.isUndef() or rhs_val.isUndef()) {
                     return sema.addConstUndef(Type.bool);
                 }
@@ -28476,9 +28487,23 @@ fn cmpNumeric(
                     return Air.Inst.Ref.bool_false;
                 }
             } else {
+                if (!lhs_val.isUndef() and (lhs_ty.isInt() or lhs_ty_tag == .ComptimeInt) and rhs_ty.isInt()) {
+                    // Compare ints: const vs. var
+                    if (sema.compareIntsOnlyPossibleResult(target, lhs_val, op, rhs_ty)) |res| {
+                        return if (res) Air.Inst.Ref.bool_true else Air.Inst.Ref.bool_false;
+                    }
+                }
                 break :src rhs_src;
             }
         } else {
+            if (try sema.resolveMaybeUndefVal(rhs)) |rhs_val| {
+                if (!rhs_val.isUndef() and (rhs_ty.isInt() or rhs_ty_tag == .ComptimeInt) and lhs_ty.isInt()) {
+                    // Compare ints: var vs. const
+                    if (sema.compareIntsOnlyPossibleResult(target, rhs_val, op.reverse(), lhs_ty)) |res| {
+                        return if (res) Air.Inst.Ref.bool_true else Air.Inst.Ref.bool_false;
+                    }
+                }
+            }
             break :src lhs_src;
         }
     };
@@ -28667,6 +28692,107 @@ fn cmpNumeric(
     return block.addBinOp(Air.Inst.Tag.fromCmpOp(op, block.float_mode == .Optimized), casted_lhs, casted_rhs);
 }
 
+/// Asserts that LHS value is an int or comptime int and not undefined, and that RHS type is an int.
+/// Given a const LHS and an unknown RHS, attempt to determine whether `op` has a guaranteed result.
+/// If it cannot be determined, returns null.
+/// Otherwise returns a bool for the guaranteed comparison operation.
+fn compareIntsOnlyPossibleResult(sema: *Sema, target: std.Target, lhs_val: Value, op: std.math.CompareOperator, rhs_ty: Type) ?bool {
+    const rhs_info = rhs_ty.intInfo(target);
+    const vs_zero = lhs_val.orderAgainstZeroAdvanced(sema) catch unreachable;
+    const is_zero = vs_zero == .eq;
+    const is_negative = vs_zero == .lt;
+    const is_positive = vs_zero == .gt;
+
+    // Anything vs. zero-sized type has guaranteed outcome.
+    if (rhs_info.bits == 0) return switch (op) {
+        .eq, .lte, .gte => is_zero,
+        .neq, .lt, .gt => !is_zero,
+    };
+
+    // Special case for i1, which can only be 0 or -1.
+    // Zero and positive ints have guaranteed outcome.
+    if (rhs_info.bits == 1 and rhs_info.signedness == .signed) {
+        if (is_positive) return switch (op) {
+            .gt, .gte, .neq => true,
+            .lt, .lte, .eq => false,
+        };
+        if (is_zero) return switch (op) {
+            .gte => true,
+            .lt => false,
+            .gt, .lte, .eq, .neq => null,
+        };
+    }
+
+    // Negative vs. unsigned has guaranteed outcome.
+    if (rhs_info.signedness == .unsigned and is_negative) return switch (op) {
+        .eq, .gt, .gte => false,
+        .neq, .lt, .lte => true,
+    };
+
+    const sign_adj = @boolToInt(!is_negative and rhs_info.signedness == .signed);
+    const req_bits = lhs_val.intBitCountTwosComp(target) + sign_adj;
+
+    // No sized type can have more than 65535 bits.
+    // The RHS type operand is either a runtime value or sized (but undefined) constant.
+    if (req_bits > 65535) return switch (op) {
+        .lt, .lte => is_negative,
+        .gt, .gte => is_positive,
+        .eq => false,
+        .neq => true,
+    };
+    const fits = req_bits <= rhs_info.bits;
+
+    // Oversized int has guaranteed outcome.
+    switch (op) {
+        .eq => return if (!fits) false else null,
+        .neq => return if (!fits) true else null,
+        .lt, .lte => if (!fits) return is_negative,
+        .gt, .gte => if (!fits) return !is_negative,
+    }
+
+    // For any other comparison, we need to know if the LHS value is
+    // equal to the maximum or minimum possible value of the RHS type.
+    const edge: struct { min: bool, max: bool } = edge: {
+        if (is_zero and rhs_info.signedness == .unsigned) break :edge .{
+            .min = true,
+            .max = false,
+        };
+
+        if (req_bits != rhs_info.bits) break :edge .{
+            .min = false,
+            .max = false,
+        };
+
+        var ty_buffer: Type.Payload.Bits = .{
+            .base = .{ .tag = if (is_negative) .int_signed else .int_unsigned },
+            .data = @intCast(u16, req_bits),
+        };
+        const ty = Type.initPayload(&ty_buffer.base);
+        const pop_count = lhs_val.popCount(ty, target);
+
+        if (is_negative) {
+            break :edge .{
+                .min = pop_count == 1,
+                .max = false,
+            };
+        } else {
+            break :edge .{
+                .min = false,
+                .max = pop_count == req_bits - sign_adj,
+            };
+        }
+    };
+
+    assert(fits);
+    return switch (op) {
+        .lt => if (edge.max) false else null,
+        .lte => if (edge.min) true else null,
+        .gt => if (edge.min) false else null,
+        .gte => if (edge.max) true else null,
+        .eq, .neq => unreachable,
+    };
+}
+
 /// Asserts that lhs and rhs types are both vectors.
 fn cmpVector(
     sema: *Sema,