[PATCH] D110867: X86InstrInfo: Support immediates that are +1/-1 different in optimizeCompareInstr

[Matthias Braun via Phabricator via llvm-commits]

MatzeB added inline comments.


================
Comment at: llvm/lib/Target/X86/X86InstrInfo.cpp:4494
+      case X86::COND_L: // x <s (C + 1)  -->  x <=s C
+        if (ImmDelta != 1 || CmpValue == INT64_MIN >> Shift)
+          return false;
----------------
MatzeB wrote:
> foad wrote:
> > Is it OK to use `INT64_MIN >> Shift` here? I think the standard says right shift of a negative value is implementation-defined.
> Good question!
> 
> I think we can rely on two-complement arithmetic shift behavior here. I'm not aware of any compiler behaving differently, certainly not the one ones specified to be used for LLVM in the documentation: https://llvm.org/docs/GettingStarted.html#host-c-toolchain-both-compiler-and-standard-library
> 
> Other code like APInt seems to rely on this as well: https://github.com/llvm/llvm-project/blob/4f0225f6d21b601d62b73dce913bf59d8fb93d87/llvm/include/llvm/ADT/APInt.h#L796
> 
> That said if you have a simple alternative to this, I'd be happy to change the code...
How about I add this:

```
static_assert(INT64_MIN >> 16 == INT32_MIN && INT64_MIN >> 24 == INT8_MIN,
              "expects compiler with twos complement right shift");
```

and then whoever has a whacky compiler will see the problem and can go fix it.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D110867/new/

https://reviews.llvm.org/D110867