diff options
Diffstat (limited to 'src/ir/effects.h')
| -rw-r--r-- | src/ir/effects.h | 17 |
1 files changed, 15 insertions, 2 deletions
diff --git a/src/ir/effects.h b/src/ir/effects.h index de228c3f4..3d71a8433 100644 --- a/src/ir/effects.h +++ b/src/ir/effects.h @@ -22,8 +22,7 @@ namespace wasm { -// Look for side effects, including control flow -// TODO: optimize +// Analyze various possible effects. class EffectAnalyzer { public: @@ -77,9 +76,23 @@ public: bool writesHeap = false; // A trap, either from an unreachable instruction, or from an implicit trap // that we do not ignore (see below). + // // Note that we ignore trap differences, so it is ok to reorder traps with // each other, but it is not ok to remove them or reorder them with other // effects in a noticeable way. + // + // Note also that we ignore runtime-dependent traps, such as hitting a + // recursion limit or running out of memory. Such traps are not part of wasm's + // official semantics, and they can occur anywhere: *any* instruction could in + // theory be implemented by a VM call (as will be the case when running in an + // interpreter), and such a call could run out of stack or memory in + // principle. To put it another way, an i32 division by zero is the program + // doing something bad that causes a trap, but the VM running out of memory is + // the VM doing something bad - and therefore the VM behaving in a way that is + // not according to the wasm semantics - and we do not model such things. Note + // that as a result we do *not* mark things like GC allocation instructions as + // having side effects, which has the nice benefit of making it possible to + // eliminate an allocation whose result is not captured. bool trap = false; // A trap from an instruction like a load or div/rem, which may trap on corner // cases. If we do not ignore implicit traps then these are counted as a trap. |