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fix 128bits ctlz intrinsincs UB #635

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fix 128bits ctlz intrinsincs UB #635

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c3a0991
fix 128bits ctlz intrinsincs UB
dvermd Feb 21, 2025
1f5f68e
add with compatible type
dvermd Feb 23, 2025
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60 changes: 26 additions & 34 deletions src/intrinsic/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -726,51 +726,43 @@ impl<'a, 'gcc, 'tcx> Builder<'a, 'gcc, 'tcx> {
"__builtin_clzll"
}
else if width == 128 {
// Algorithm from: https://stackoverflow.com/a/28433850/389119
let array_type = self.context.new_array_type(None, arg_type, 3);
let result = self.current_func()
.new_local(None, array_type, "count_loading_zeroes_results");

// Algorithm from: https://stackoverflow.com/a/28433850/389119 updated to check for high 64bits being 0
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@antoyo antoyo Mar 28, 2025 •
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It seems your new algorithm significantly diverges from the one linked here.
If so, could you please replace this link with another pointing to the algorithm you used? If you can't, could you please add comments below to make this easier to understand?

let result = self.current_func().new_local(None, result_type, "zeros");
let ctlz_then_block = self.current_func().new_block("ctlz_then");
let ctlz_else_block = self.current_func().new_block("ctlz_else");
let ctlz_after_block = self.current_func().new_block("ctlz_after");
let sixty_four = self.const_uint(arg_type, 64);
let shift = self.lshr(arg, sixty_four);
let high = self.gcc_int_cast(shift, self.u64_type);
let low = self.gcc_int_cast(arg, self.u64_type);

let zero = self.context.new_rvalue_zero(self.usize_type);
let one = self.context.new_rvalue_one(self.usize_type);
let two = self.context.new_rvalue_from_long(self.usize_type, 2);

let clzll = self.context.get_builtin_function("__builtin_clzll");

let first_elem = self.context.new_array_access(None, result, zero);
let first_value = self.gcc_int_cast(self.context.new_call(None, clzll, &[high]), arg_type);
self.llbb()
.add_assignment(self.location, first_elem, first_value);
let zero = self.const_uint(high.get_type(), 0);
let cond = self.gcc_icmp(IntPredicate::IntNE, high, zero);
self.llbb().end_with_conditional(
self.location,
cond,
ctlz_then_block,
ctlz_else_block,
);

let second_elem = self.context.new_array_access(self.location, result, one);
let cast = self.gcc_int_cast(self.context.new_call(self.location, clzll, &[low]), arg_type);
let second_value = self.add(cast, sixty_four);
self.llbb()
.add_assignment(self.location, second_elem, second_value);
let leading_zeroes =
self.gcc_int_cast(self.context.new_call(None, clzll, &[high]), result_type);

let third_elem = self.context.new_array_access(self.location, result, two);
let third_value = self.const_uint(arg_type, 128);
self.llbb()
.add_assignment(self.location, third_elem, third_value);
ctlz_then_block.add_assignment(None, result, leading_zeroes);
ctlz_then_block.end_with_jump(None, ctlz_after_block);

let not_high = self.context.new_unary_op(self.location, UnaryOp::LogicalNegate, self.u64_type, high);
let not_low = self.context.new_unary_op(self.location, UnaryOp::LogicalNegate, self.u64_type, low);
let not_low_and_not_high = not_low & not_high;
let index = not_high + not_low_and_not_high;
// NOTE: the following cast is necessary to avoid a GIMPLE verification failure in
// gcc.
// TODO(antoyo): do the correct verification in libgccjit to avoid an error at the
// compilation stage.
let index = self.context.new_cast(self.location, index, self.i32_type);
let low = self.gcc_int_cast(arg, self.u64_type);
let low_leading_zeroes =
self.gcc_int_cast(self.context.new_call(None, clzll, &[low]), result_type);
let sixty_four_u32 = self.const_uint(result_type, 64);
let leading_zeroes = self.add(low_leading_zeroes, sixty_four_u32);
ctlz_else_block.add_assignment(None, result, leading_zeroes);
ctlz_else_block.end_with_jump(None, ctlz_after_block);

let res = self.context.new_array_access(self.location, result, index);
self.switch_to_block(ctlz_after_block);

return self.gcc_int_cast(res.to_rvalue(), result_type);
return result.to_rvalue();
}
else {
let count_leading_zeroes = self.context.get_builtin_function("__builtin_clzll");
Expand Down