[PATCH] D45673: [x86] Fix PR37100 by teaching the EFLAGS copy lowering to rewrite uses across basic blocks in the limited cases where it is very straight forward to do so.

[Reid Kleckner via Phabricator via llvm-commits]

rnk added inline comments.


================
Comment at: llvm/test/CodeGen/X86/copy-eflags.ll:293
+  %tmp11 = trunc i32 %tmp10 to i16
+  br label %bb1
+}
----------------
This infinite loop with selects of undef feels over-reduced. It seems fragile, since a pass could come along and optimize the select to the non-undef operand.

I liked the test case from PR37133, but it's reliance on debug info (any, not just standalone) scares me:
```
int a, b;
unsigned long long c;
void e() {
  long long d;
  b = 33;
  d = a;
  if (c < a) {
    b = 32;
    d = c;
  }
  if (d)
    b = 2;
}
```

We should reopen it or file a split separate bug, because we generate two more BBs on that code when debug info is enabled. :(

In any case, is there a better way to write this test case using legalized arithmetic? An i128 x64 test case with live-out eflags might also be interesting.


Repository:
  rL LLVM

https://reviews.llvm.org/D45673