<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/104827>104827</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            Subtracting from an unsigned number and then rounding down should know the result is less than the original
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            new issue
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          shepmaster
      </td>
    </tr>
</table>

<pre>
    I have a Rust byte slice where I am trying to divide it into an even multiple of 64-byte chunks. The extra wrinkle is that I want to have at least one full chunk leftover, so I'm subtracting 64 bytes right at the beginning:

```rust
pub fn x(a: &[u8]) {
    if let Some(len) = a.len().checked_sub(64) {
        let len = len / 64 * 64;
 a.split_at(len);
    }
}
```

The argument to `split_at` should always be less than the input slice's length, so this can never panic, but the resulting optimized code still contains a call to panic. This code can be simplified down to plain numbers:

```rust
pub fn mini(length: u8) -> bool {
    if let Some(l) = length.checked_sub(64) {
        let cut = (l / 64) * 64;
        // Or this alternate formulation
        //let cut = l & !(64 - 1);

        return cut <= length;
    }

    true
}
```

Which produces this LLVM IR:

```llvm
; ModuleID = 'example.32ce4472ca1d966e-cgu.0'
source_filename = "example.32ce4472ca1d966e-cgu.0"
target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"

; Function Attrs: mustprogress nofree norecurse nosync nounwind nonlazybind willreturn memory(none) uwtable
define noundef zeroext i1 @mini(i8 noundef %length) unnamed_addr #0 {
start:
  %_6 = icmp ult i8 %length, 64
  %0 = and i8 %length, -64
  %_51 = add i8 %0, -64
  %1 = icmp ule i8 %_51, %length
 %_0.sroa.0.0 = or i1 %_6, %1
  ret i1 %_0.sroa.0.0
}

attributes #0 = { mustprogress nofree norecurse nosync nounwind nonlazybind willreturn memory(none) uwtable "probe-stack"="inline-asm" "target-cpu"="x86-64" }

!llvm.module.flags = !{!0, !1}
!llvm.ident = !{!2}

!0 = !{i32 8, !"PIC Level", i32 2}
!1 = !{i32 2, !"RtLibUseGOT", i32 1}
!2 = !{!"rustc version 1.82.0-nightly (6de928dce 2024-08-18)"}
```

However, this code should be able to be simplified down to `return true`, and [alive2 agrees](https://alive2.llvm.org/ce/z/yzuYvR).

Similar in spirit to #91527 
</pre>
<img width="1px" height="1px" alt="" src="http://email.email.llvm.org/o/eJy0Vl2P47rN_jXKDeHAlh3HucjFfLzzdoA9OMXuaYteDWSbttWRJUMfyWR-fUHZ-drtnu5NBwNHth5SJB9SpHBO9hpxzzaPbPO8EsEPxu7dgNMonEe7qk172r_CIA4IAr4G56E-eQSnZINwHNAivIIYwduT1D14A608yBZBepDaGxAa8IAaxqC8nBSC6aAskqilGYJ-d2v4Y0DAD28FHK3U7wpBOvCD8PAKR6E9qZ1N8KBQOA9GI3RBqVkFKOy8OaBl_AmcgVfGtyO4UHsrGk92lUW024GV_eBJjx8Qauyl1lL3LH9g6TNLz88ynf9tcH7-NIUaOg0fjFeC5Q_AeMk2j6Fim2fGd8C2jzMOAEB2oNDDNzMi45VCHRH5M4h1fKkY362bAZt3bN9cqBmvyuIHLfRHahTqKBx_-Qu5wvgDlAXLz2ixdpOS_k34y3nXTQBg2-fFscvi7OGt20SDsH0YcQ45K9OL3jIFN5igWhDqKE4OagSFLtKkYzClnoKfE4PxrSN7ez8sjPhBOmiEBo0HtDAJLRvaqsPMhEVH-aF7MJOXo_zEFhrTIjgviWWjvZDagYBGKEXGRRWUOqSYkKS9RnBynJTsJLbQmqOOUCWkBh3GGq37ZapHqeUcTnIjf4BAvEHC8v-D2hj1p5yfGZ-lf53sJvgoRzoWtiP2O8KXP8ZfCPO7neMrlEerhUfojB2DEl4a_Z8kbk-iY0pgPIuGQQLZbfbcS1v0wepF9Onq30-S7fLJ24C_koD_GGQzwGRNGxp0s1Nfvvz9N3j9-jPalDqMy6f8EX4zbVD4-rzEcIsfYpwUrnPeYFFseSOydleWmDR9WKeMb2dRZ4Jt8K2TCrUYcZHm_02az9Je2B49tMILJU7mwiDHZGT5AyYT36Ysf8h5fNBrdv9KK6L3oSwSeVlkvGL5Q8arpKvSZaXjp_IsXhbJN4J9Z4q38aZdzPioyreySIJ-1-aoEyV1-Eh6HS5S1_i9BN1Q0sCD97FUYAzOT9b0lmpdm84igjYWm2AdrdxJN6BN0EepW9BGK_F5qml9lEotCTPiaOyJ8UobjZTP4ehFrZacaLGTGqOSFjv4RGvww4PMgBXpUoWyuuwzvjlfLTsImghr30TbWmA8T69l5byw_pI2lPqbtzLGRDbjBEF5kNWtticqsSs4na9s3f4AS-5wb5tsRrZnZPojKLs9GBfc2yYj5FX3jKeddO2sEet0PVthbAwHebBIZGflFv157yr0fbHFp_DeyjpQF5wjRemxffzfUkwJOFlTY-K8aN4p5_JnxrnUSmpMhBsZ5wSaUzdppnDBfFQlRZH27z1hPKO6X4-x3NedEr1bsj0j_nmWzmHKsqvkIiNb6m53YP6j-vQGIXMO1aKPcf7X1yf4ggdUZCd_Atrmt8dk38nyq-xX_0XWf3P4_7__cSN9ZyS_t41xTp2pgQNaR5WZrSu-ThNNc4w6UaMoW9zxqm0QeMqLJK2SjDoVHfCnt-1fzBGXmclfuujS5GuEyJ43P-mp1DFn5uPlXsaAU7GwzaNQ8oAcRG8RXRyQqsH7Kbbe2H9mwDryYWzP-AvNDC-fjL-cPsM_D19pQLo19ZscpRIWpAY3SSvn8YTnu2zDt7Bq93m7y3dihftsy4t0l6W73WrY52lR53WRiZa3YlfUGe445s0uFy3fbJt2JfcUsrTKdlmVZflunW2bLqsynrV513UZsiLFUUh1sXUlnQu4z9Ki4tuVEjUqF8dnzjUeIe5S5DfPK7snoaQOvWNFqqTz7qrGS69w_-1mRO2sGWlaDjoO5e0ysMSg-gE1WLoACRk5WIiiO_1mgqLB-X4sM1b2Ugu1Clbt72nopR9CvW7MSDMBNdL5J5ms-Rc2nvGX6I9j_GVx-LDn_w4AAP__a9KS9g">