[ValueTracking] don't delete assumes of side-effectful instructions

[arielb1]

Summary

ValueTracking has to strike a balance when attempting to propagate information backwards from assumes, because if the information is trivially propagated backwards, it can appear to LLVM that the assumption is known to be true, and therefore can be removed.

This is sound (because an assumption has no semantic effect except for causing UB), but prevents the assume from allowing further optimizations.

The isEphemeralValueOf check exists to try and prevent this issue by not removing the source of an assumption. This tries to make it a little bit more general to handle the case of side-effectful instructions, such as in

%0 = call i1 @get_val()
%1 = xor i1 %0, true
call void @llvm.assume(i1 %1)

FIX: try to make llvm-commits see this

[hfinkel]

LGTM

llvm/trunk/lib/Analysis/ValueTracking.cpp

while (!WorkSet.empty()) {
     if (!Visited.insert(V).second)
       continue;
 
     // If all uses of this value are ephemeral, then so is this value.
     if (all_of(V->users(), [&](const User *U) { return EphValues.count(U); })) {
       if (V == E)
         return true;
 
+      if (V == I || isSafeToSpeculativelyExecute(V)) {
        EphValues.insert(V);
        if (const User *U = dyn_cast<User>(V))
          for (User::const_op_iterator J = U->op_begin(), JE = U->op_end();
-             J != JE; ++J) {
-          if (isSafeToSpeculativelyExecute(*J))
-            WorkSet.push_back(*J);
-        }
+              J != JE; ++J)
     }
   }
 
   return false;
 }
 
 // Is this an intrinsic that cannot be speculated but also cannot trap?
 static bool isAssumeLikeIntrinsic(const Instruction *I) {

llvm/trunk/test/Analysis/ValueTracking/assume.ll

;
   %t1 = add i32 %a, %b
   %last_two_digits = and i32 %t1, 3
   %t2 = icmp eq i32 %last_two_digits, 0
   call void @llvm.assume(i1 %t2)
   %t3 = add i32 %t1, 3
   ret i32 %t3
 }
 
-declare void @llvm.assume(i1)
 
+define void @assume_not() {
+; CHECK-LABEL: @assume_not(
+entry-block:
+  %0 = call i1 @get_val()
+; CHECK: call void @llvm.assume
+  %1 = xor i1 %0, true
+  call void @llvm.assume(i1 %1)
+  ret void
+}
+
+declare i1 @get_val()
+declare void @llvm.assume(i1)