[AArch64] Inefficient loading of some immediates

[AZero13]

https://godbolt.org/z/Yo55Gz8eP

We decompose many immediates, which is a good thing. However, there are some that we decompose that we can just avoid with the mov instruction. This is one such example.

[AZero13]

https://godbolt.org/z/anGPGEeWh More simplified.

[AZero13]

Basically:

check1:
mov w8, #65534
movk w8, #65279, lsl #16
cmp w0, w8
cset w0, hi
ret

But we can do:
check1:
mov w1, -16777217
cmp w0, w1
cset w0, cs
ret

[llvmbot]

@llvm/issue-subscribers-backend-aarch64

Author: AZero13 (AZero13)

https://godbolt.org/z/Yo55Gz8eP

We decompose many immediates, which is a good thing. However, there are some that we decompose that we can just avoid with the mov instruction. This is one such example.

[pinskia]

Note GCC implemented this in GCC 9 [generic non-aarch64 specific path] (and GCC 15 for the ccmp path):
https://gcc.gnu.org/r9-2428 and https://gcc.gnu.org/r15-4759 .

[efriedma-quic]

We have code for adjusting condition codes to simplify cmp immediates; see https://github.com/llvm/llvm-project/blob/26d9cb17a6e655993c991b66b21d5c378256d79b/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp#L4077 . We currently don't try to do any adjustments unless the adjustment results in a legal cmp immediate.

gcc apparently misses the adjustment for the following.

int check1 (int x)
{
  return x > 0xaa9fff;
}

[pinskia]

gcc apparently misses the adjustment for the following.

int check1 (int x)
{
  return x > 0xaa9fff;
}

I see the issue, the problem is how GCC calculates the cost here. https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120372 (fix included which I will be submitting soon).

[AZero13]

We have code for adjusting condition codes to simplify cmp immediates; see https://github.com/llvm/llvm-project/blob/26d9cb17a6e655993c991b66b21d5c378256d79b/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp#L4077 . We currently don't try to do any adjustments unless the adjustment results in a legal cmp immediate.

gcc apparently misses the adjustment for the following.

int check1 (int x)
{
  return x > 0xaa9fff;
}

fixing that in particular is not an easy task because by the time the ccmp is generated, so is the cmp, which is another bug I meant to report but forgot to.