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Add llvm.sideeffect to potential infinite loops and recursions

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LLVM assumes that a thread will eventually cause side effect. This is
not true in Rust if a loop or recursion does nothing in its body,
causing undefined behavior even in common cases like `loop {}`.
Inserting llvm.sideeffect fixes the undefined behavior.

As a micro-optimization, only insert llvm.sideeffect when jumping back
in blocks or calling a function.

A patch for LLVM is expected to allow empty non-terminate code by
default and fix this issue from LLVM side.

https://github.com/rust-lang/rust/issues/28728
This commit is contained in:
Xiang Fan 2019-06-06 08:39:20 +08:00
parent a37fe2de69
commit f71e0daa29
18 changed files with 127 additions and 18 deletions
Download patch file Download diff file Expand all files Collapse all files

36
src/librustc_codegen_ssa/mir/block.rs
View file

@ -148,6 +148,24 @@ impl<'a, 'tcx> TerminatorCodegenHelper<'a, 'tcx> {
}
}
}
// Generate sideeffect intrinsic if jumping to any of the targets can form
// a loop.
fn maybe_sideeffect<'b, 'tcx2: 'b, Bx: BuilderMethods<'b, 'tcx2>>(
&self,
mir: &'b mir::Body<'tcx>,
bx: &mut Bx,
targets: &[mir::BasicBlock],
) {
if targets.iter().any(|target| {
*target <= *self.bb
&& target
.start_location()
.is_predecessor_of(self.bb.start_location(), mir)
}) {
bx.sideeffect();
}
}
}
/// Codegen implementations for some terminator variants.
@ -196,6 +214,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
let lltrue = helper.llblock(self, targets[0]);
let llfalse = helper.llblock(self, targets[1]);
if switch_ty == bx.tcx().types.bool {
helper.maybe_sideeffect(self.mir, &mut bx, targets.as_slice());
// Don't generate trivial icmps when switching on bool
if let [0] = values[..] {
bx.cond_br(discr.immediate(), llfalse, lltrue);
@ -209,9 +228,11 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
);
let llval = bx.const_uint_big(switch_llty, values[0]);
let cmp = bx.icmp(IntPredicate::IntEQ, discr.immediate(), llval);
helper.maybe_sideeffect(self.mir, &mut bx, targets.as_slice());
bx.cond_br(cmp, lltrue, llfalse);
}
} else {
helper.maybe_sideeffect(self.mir, &mut bx, targets.as_slice());
let (otherwise, targets) = targets.split_last().unwrap();
bx.switch(
discr.immediate(),
@ -310,6 +331,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
if let ty::InstanceDef::DropGlue(_, None) = drop_fn.def {
// we don't actually need to drop anything.
helper.maybe_sideeffect(self.mir, &mut bx, &[target]);
helper.funclet_br(self, &mut bx, target);
return
}
@ -340,6 +362,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
FnType::of_instance(&bx, drop_fn))
}
};
bx.sideeffect();
helper.do_call(self, &mut bx, fn_ty, drop_fn, args,
Some((ReturnDest::Nothing, target)),
unwind);
@ -375,6 +398,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
// Don't codegen the panic block if success if known.
if const_cond == Some(expected) {
helper.maybe_sideeffect(self.mir, &mut bx, &[target]);
helper.funclet_br(self, &mut bx, target);
return;
}
@ -385,6 +409,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
// Create the failure block and the conditional branch to it.
let lltarget = helper.llblock(self, target);
let panic_block = self.new_block("panic");
helper.maybe_sideeffect(self.mir, &mut bx, &[target]);
if expected {
bx.cond_br(cond, lltarget, panic_block.llbb());
} else {
@ -437,6 +462,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
let fn_ty = FnType::of_instance(&bx, instance);
let llfn = bx.get_fn(instance);
bx.sideeffect();
// Codegen the actual panic invoke/call.
helper.do_call(self, &mut bx, fn_ty, llfn, &args, None, cleanup);
}
@ -488,6 +514,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
if let Some(destination_ref) = destination.as_ref() {
let &(ref dest, target) = destination_ref;
self.codegen_transmute(&mut bx, &args[0], dest);
helper.maybe_sideeffect(self.mir, &mut bx, &[target]);
helper.funclet_br(self, &mut bx, target);
} else {
// If we are trying to transmute to an uninhabited type,
@ -518,6 +545,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
Some(ty::InstanceDef::DropGlue(_, None)) => {
// Empty drop glue; a no-op.
let &(_, target) = destination.as_ref().unwrap();
helper.maybe_sideeffect(self.mir, &mut bx, &[target]);
helper.funclet_br(self, &mut bx, target);
return;
}
@ -554,6 +582,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
let fn_ty = FnType::of_instance(&bx, instance);
let llfn = bx.get_fn(instance);
bx.sideeffect();
// Codegen the actual panic invoke/call.
helper.do_call(
self,
@ -566,7 +595,9 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
);
} else {
// a NOP
helper.funclet_br(self, &mut bx, destination.as_ref().unwrap().1)
let target = destination.as_ref().unwrap().1;
helper.maybe_sideeffect(self.mir, &mut bx, &[target]);
helper.funclet_br(self, &mut bx, target);
}
return;
}
@ -675,6 +706,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
}
if let Some((_, target)) = *destination {
helper.maybe_sideeffect(self.mir, &mut bx, &[target]);
helper.funclet_br(self, &mut bx, target);
} else {
bx.unreachable();
@ -786,6 +818,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
_ => span_bug!(span, "no llfn for call"),
};
bx.sideeffect();
helper.do_call(self, &mut bx, fn_ty, fn_ptr, &llargs,
destination.as_ref().map(|&(_, target)| (ret_dest, target)),
cleanup);
@ -835,6 +868,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
}
mir::TerminatorKind::Goto { target } => {
helper.maybe_sideeffect(self.mir, &mut bx, &[target]);
helper.funclet_br(self, &mut bx, target);
}

1
src/librustc_codegen_ssa/mir/rvalue.rs
View file

@ -486,6 +486,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
};
let instance = ty::Instance::mono(bx.tcx(), def_id);
let r = bx.cx().get_fn(instance);
bx.sideeffect();
let call = bx.call(r, &[llsize, llalign], None);
let val = bx.pointercast(call, llty_ptr);

1
src/librustc_codegen_ssa/traits/intrinsic.rs
View file

@ -20,6 +20,7 @@ pub trait IntrinsicCallMethods<'tcx>: BackendTypes {
fn abort(&mut self);
fn assume(&mut self, val: Self::Value);
fn expect(&mut self, cond: Self::Value, expected: bool) -> Self::Value;
fn sideeffect(&mut self);
/// Trait method used to inject `va_start` on the "spoofed" `VaListImpl` in
/// Rust defined C-variadic functions.
fn va_start(&mut self, val: Self::Value) -> Self::Value;