fuzzgen: Add fcvt_* ops (#4958)

cranelift/fuzzgen/src/passes/fcvt.rs

@@ -0,0 +1,98 @@
+use crate::{FuzzGen, Type};
+use anyhow::Result;
+use cranelift::codegen::cursor::{Cursor, FuncCursor};
+use cranelift::codegen::ir::{Function, Inst, Opcode};
+use cranelift::prelude::{types::*, *};
+
+pub fn do_fcvt_trap_pass(fuzz: &mut FuzzGen, func: &mut Function) -> Result<()> {
+    let ratio = fuzz.config.allowed_fcvt_traps_ratio;
+    let insert_seq = !fuzz.u.ratio(ratio.0, ratio.1)?;
+    if !insert_seq {
+        return Ok(());
+    }
+
+    let mut pos = FuncCursor::new(func);
+    while let Some(_block) = pos.next_block() {
+        while let Some(inst) = pos.next_inst() {
+            if can_fcvt_trap(&pos, inst) {
+                insert_fcvt_sequence(&mut pos, inst);
+            }
+        }
+    }
+    Ok(())
+}
+
+/// Returns true/false if this instruction can trap
+fn can_fcvt_trap(pos: &FuncCursor, inst: Inst) -> bool {
+    let opcode = pos.func.dfg[inst].opcode();
+
+    matches!(opcode, Opcode::FcvtToUint | Opcode::FcvtToSint)
+}
+
+/// Gets the max and min float values for this integer type
+/// Inserts fconst instructions with these values.
+//
+// When converting to integers, floats are truncated. This means that the maximum float value
+// that can be converted into an i8 is 127.99999. And surprisingly the minimum float for an
+// u8 is -0.99999! So get the limits of this type as a float value by adding or subtracting
+// 1.0 from its min and max integer values.
+fn float_limits(
+    pos: &mut FuncCursor,
+    float_ty: Type,
+    int_ty: Type,
+    is_signed: bool,
+) -> (Value, Value) {
+    let (min_int, max_int) = int_ty.bounds(is_signed);
+
+    if float_ty == F32 {
+        let (min, max) = if is_signed {
+            ((min_int as i128) as f32, (max_int as i128) as f32)
+        } else {
+            (min_int as f32, max_int as f32)
+        };
+
+        (pos.ins().f32const(min - 1.0), pos.ins().f32const(max + 1.0))
+    } else {
+        let (min, max) = if is_signed {
+            ((min_int as i128) as f64, (max_int as i128) as f64)
+        } else {
+            (min_int as f64, max_int as f64)
+        };
+
+        (pos.ins().f64const(min - 1.0), pos.ins().f64const(max + 1.0))
+    }
+}
+
+/// Prepend instructions to inst to avoid traps
+fn insert_fcvt_sequence(pos: &mut FuncCursor, inst: Inst) {
+    let dfg = &pos.func.dfg;
+    let opcode = dfg[inst].opcode();
+    let arg = dfg.inst_args(inst)[0];
+    let float_ty = dfg.value_type(arg);
+    let int_ty = dfg.value_type(dfg.first_result(inst));
+
+    // These instructions trap on NaN
+    let is_nan = pos.ins().fcmp(FloatCC::NotEqual, arg, arg);
+
+    // They also trap if the value is larger or smaller than what the integer type can represent. So
+    // we generate the maximum and minimum float value that would make this trap, and compare against
+    // those limits.
+    let is_signed = opcode == Opcode::FcvtToSint;
+    let (min, max) = float_limits(pos, float_ty, int_ty, is_signed);
+    let underflows = pos.ins().fcmp(FloatCC::LessThanOrEqual, arg, min);
+    let overflows = pos.ins().fcmp(FloatCC::GreaterThanOrEqual, arg, max);
+
+    // Check the previous conditions and replace with a 1.0 if this instruction would trap
+    let overflows_int = pos.ins().bor(underflows, overflows);
+    let is_invalid = pos.ins().bor(is_nan, overflows_int);
+
+    let one = if float_ty == F32 {
+        pos.ins().f32const(1.0)
+    } else {
+        pos.ins().f64const(1.0)
+    };
+    let new_arg = pos.ins().select(is_invalid, one, arg);
+
+    // Replace the previous arg with the new one
+    pos.func.dfg.inst_args_mut(inst)[0] = new_arg;
+}

cranelift/fuzzgen/src/passes/int_divz.rs

@@ -0,0 +1,78 @@
+use crate::FuzzGen;
+use anyhow::Result;
+use cranelift::codegen::cursor::{Cursor, FuncCursor};
+use cranelift::codegen::ir::{Function, Inst, Opcode};
+use cranelift::prelude::{InstBuilder, IntCC};
+
+pub fn do_int_divz_pass(fuzz: &mut FuzzGen, func: &mut Function) -> Result<()> {
+    // Insert this per function, otherwise the actual rate of int_divz doesn't go down that much
+    // Experimentally if we decide this per instruction with a 0.1% allow rate, we get 4.4% of runs
+    // trapping. Doing this per function decreases the number of runs that trap. It also consumes
+    // fewer fuzzer input bytes which is nice.
+    let ratio = fuzz.config.allowed_int_divz_ratio;
+    let insert_seq = !fuzz.u.ratio(ratio.0, ratio.1)?;
+    if !insert_seq {
+        return Ok(());
+    }
+
+    let mut pos = FuncCursor::new(func);
+    while let Some(_block) = pos.next_block() {
+        while let Some(inst) = pos.next_inst() {
+            if can_int_divz(&pos, inst) {
+                insert_int_divz_sequence(&mut pos, inst);
+            }
+        }
+    }
+    Ok(())
+}
+
+/// Returns true/false if this instruction can cause a `int_divz` trap
+fn can_int_divz(pos: &FuncCursor, inst: Inst) -> bool {
+    let opcode = pos.func.dfg[inst].opcode();
+
+    matches!(
+        opcode,
+        Opcode::Sdiv | Opcode::Udiv | Opcode::Srem | Opcode::Urem
+    )
+}
+
+/// Prepend instructions to inst to avoid `int_divz` traps
+fn insert_int_divz_sequence(pos: &mut FuncCursor, inst: Inst) {
+    let opcode = pos.func.dfg[inst].opcode();
+    let inst_args = pos.func.dfg.inst_args(inst);
+    let (lhs, rhs) = (inst_args[0], inst_args[1]);
+    assert_eq!(pos.func.dfg.value_type(lhs), pos.func.dfg.value_type(rhs));
+    let ty = pos.func.dfg.value_type(lhs);
+
+    // All of these instructions can trap if the denominator is zero
+    let zero = pos.ins().iconst(ty, 0);
+    let one = pos.ins().iconst(ty, 1);
+    let denominator_is_zero = pos.ins().icmp(IntCC::Equal, rhs, zero);
+
+    let replace_denominator = if matches!(opcode, Opcode::Srem | Opcode::Sdiv) {
+        // Srem and Sdiv can also trap on INT_MIN / -1. So we need to check for the second one
+
+        // 1 << (ty bits - 1) to get INT_MIN
+        let int_min = pos.ins().ishl_imm(one, ty.lane_bits() as i64 - 1);
+
+        // Get a -1 const
+        // TODO: A iconst -1 would be clearer, but #2906 makes this impossible for i128
+        let neg_one = pos.ins().isub(zero, one);
+
+        let lhs_check = pos.ins().icmp(IntCC::Equal, lhs, int_min);
+        let rhs_check = pos.ins().icmp(IntCC::Equal, rhs, neg_one);
+        let is_invalid = pos.ins().band(lhs_check, rhs_check);
+
+        // These also crash if the denominator is zero, so we still need to check for that.
+        pos.ins().bor(denominator_is_zero, is_invalid)
+    } else {
+        denominator_is_zero
+    };
+
+    // If we have a trap we replace the denominator with a 1
+    let new_rhs = pos.ins().select(replace_denominator, one, rhs);
+
+    // Replace the previous rhs with the new one
+    let args = pos.func.dfg.inst_args_mut(inst);
+    args[1] = new_rhs;
+}

cranelift/fuzzgen/src/passes/mod.rs

@@ -0,0 +1,5 @@
+mod fcvt;
+mod int_divz;
+
+pub use fcvt::do_fcvt_trap_pass;
+pub use int_divz::do_int_divz_pass;