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LLVM srem is undefined in cases where i32.rem_s and i64.rem_s are defined.

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Nick Lewycky 2019-07-24 17:17:06 -07:00 committed by Nick Lewycky
parent f483c89da7
commit 2f021f0e2a
1 changed files with 36 additions and 0 deletions
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36
lib/llvm-backend/src/code.rs
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@ -1577,9 +1577,45 @@ impl FunctionCodeGenerator<CodegenError> for LLVMFunctionCodeGenerator {
Operator::I32RemS | Operator::I64RemS => {
let (v1, v2) = state.pop2()?;
let (v1, v2) = (v1.into_int_value(), v2.into_int_value());
let int_type = v1.get_type();
let (min_value, neg_one_value) = if int_type == intrinsics.i32_ty {
let min_value = int_type.const_int(i32::min_value() as u64, false);
let neg_one_value = int_type.const_int(-1i32 as u32 as u64, false);
(min_value, neg_one_value)
} else if int_type == intrinsics.i64_ty {
let min_value = int_type.const_int(i64::min_value() as u64, false);
let neg_one_value = int_type.const_int(-1i64 as u64, false);
(min_value, neg_one_value)
} else {
unreachable!()
};
trap_if_zero(builder, intrinsics, context, &function, v2);
// "Overflow also leads to undefined behavior; this is a rare
// case, but can occur, for example, by taking the remainder of
// a 32-bit division of -2147483648 by -1. (The remainder
// doesn’t actually overflow, but this rule lets srem be
// implemented using instructions that return both the result
// of the division and the remainder.)"
// -- https://llvm.org/docs/LangRef.html#srem-instruction
//
// In Wasm, the i32.rem_s i32.const -2147483648 i32.const -1 is
// i32.const 0. We implement this by swapping out the left value
// for 0 in this case.
let will_overflow = builder.build_and(
builder.build_int_compare(IntPredicate::EQ, v1, min_value, "left_is_min"),
builder.build_int_compare(
IntPredicate::EQ,
v2,
neg_one_value,
"right_is_neg_one",
),
"srem_will_overflow",
);
let v1 = builder
.build_select(will_overflow, int_type.const_zero(), v1, "")
.into_int_value();
let res = builder.build_int_signed_rem(v1, v2, &state.var_name());
state.push1(res);
}