From ae11c3f83e92d776b9f24a569532229ba106b4b0 Mon Sep 17 00:00:00 2001
From: Dan Gohman
Any non-void instruction or constant expression other than a non-intrinsic - call, invoke, or phi with a trap operand has trap as its result value. - Any instruction with a trap operand which may have side effects emits - those side effects as if it had an undef operand instead.
+Any value other than a non-intrinsic call, invoke, or phi with a trap + operand has trap as its result value. Any instruction with + a trap operand which may have side effects emits those side effects as + if it had an undef operand instead. If the side effects are externally + visible, the behavior is undefined.
+ +Trap values may be stored to memory; a load from memory including any + part of a trap value results in a (full) trap value.
+ +For example:
+ + + ++%trap = sub nuw i32 0, 1 ; Results in a trap value. +%still_trap = and i32 %trap, 0 ; Whereas (and i32 undef, 0) would return 0. +%trap_yet_again = getelementptr i32* @h, i32 %still_trap +store i32 0, i32* %trap_yet_again ; undefined behavior + +volatile store i32 %trap, i32* @g ; External observation; undefined behavior. +%trap2 = load i32* @g ; Returns a trap value, not just undef. +%narrowaddr = bitcast i32* @g to i16* +%wideaddr = bitcast i32* @g to i64* +%trap3 = load 16* %narrowaddr ; Returns a trap value +%trap4 = load i64* %widaddr ; Returns a trap value, not partial trap. ++
If a br or switch instruction has a trap value operand, all non-phi non-void instructions which control-depend on it - have trap as their result value. If any instruction which - control-depends on the br or switch invokes externally - visible side effects, the behavior of the program is undefined.
+ have trap as their result value. A phi + node with an incoming value associated with a control edge which is + control-dependent on it has trap as its result value when control is + transferred from that block. If any instruction which control-depends + on the br or switch invokes externally visible side + effects, the behavior of the program is undefined. For example: -For example, an and of a trap value with - zero still has a trap value result. Using that value as an index in a - getelementptr yields a trap - result. Using that result as the address of a - store produces undefined behavior.
++entry: + %trap = sub nuw i32 0, 1 ; Results in a trap value. + %cmp = icmp i32 slt %trap, 0 ; Still trap. + %br i1 %cmp, %true, %end ; Branch to either destination. + +true: + volatile store i32 0, i32* @g ; Externally visible side effects + ; control-dependent on %cmp. + ; Undefined behavior. + br label %end + +end: + %p = phi i32 [ 0, %entry ], [ 1, %true ] + ; Both edges into this PHI are + ; control-dependent on %cmp, so this + ; results in a trap value. + + volatile store i32 0, i32* @g ; %end is control-equivalent to %entry + ; so this is defined (ignoring earlier + ; undefined behavior in this example). + ++
There is currently no way of representing a trap constant in the IR; they only exist when produced by certain instructions, such as an -- cgit v1.2.3