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LJ 32763fedf2 Fix Windows bytecode divergence during LLVM LSR (#570 )

## Description

Our differential bytecode comparison tests detected a bytecode mismatch
between the Linux/macOS builds and the Windows build for a
[contract](https://github.com/paritytech/resolc-compiler-tests/blob/4647018d64eab6ccff30d22c753f02b90e9fc7e3/fixtures/solidity/simple/internal_function_pointers/sort/bubble.sol)
implementing recursive bubble sort, specifically with these opt
settings:
* Solc optimizer disabled
* Newyork enabled
* `-Oz`

This PR applies an [LLVM
patch](https://github.com/llvm/llvm-project/pull/190587) which fixes the
divergence for contracts with loops that index array elements at
constant offsets from a Solidity `uint8` loop variable, observed with
the above opt settings.

## Cause

The first divergence occurs in an LLVM Loop Strength Reduction (LSR)
decision, on `bubbleSort`'s loop (`for (uint8 i = 0; i < ARRAY_SIZE -
start - 1; i++)` with `array[i]`/`array[i+1]` accesses):

```diff
-  %lsr.iv = phi i32 [ 0, %for_cond.preheader153 ], [ %lsr.iv.next, %if_join64 ]   ; Linux/macOS
+  %lsr.iv = phi i32 [ 1, %for_cond.preheader153 ], [ %lsr.iv.next, %if_join64 ]   ; Windows
+  %3 = add i32 %lsr.iv, -1                                                        ; Windows
 ...
-  %3 = add i32 %lsr.iv, 1                                                         ; Linux/macOS
 ...
   %lsr.iv.next = add nuw nsw i32 %lsr.iv, 1   ; identical on both platforms: step is +1 either way
```

In LLVM's `{Start,+,Step}` recurrence notation (value starts at `Start`,
changes by `Step` per loop iteration), the three constants in the diff
represent: recurrence *start*, use-site *offset*, and recurrence *step*.
Only the first two differ between the platforms:

**Linux/macOS** (the `array[i]` calls take `%lsr.iv` directly; the
`array[i+1]` calls take `%3`):

| Line | Constant | Role |
|---|---|---|
| `%lsr.iv = phi i32 [ 0, … ]` | `0` | **Start**: the register's value
on loop entry (`i`) (holds 0, 1, 2, …) |
| `%3 = add i32 %lsr.iv, 1` | `+1` | **Use-site offset**: derives the
*other* index (`i + 1`) from the register each iteration. (No register
is dedicated to `%3`) |
| `%lsr.iv.next = add … , 1` | `+1` | **Step**: how much the register
advances each iteration |

**Windows** (mirrored: the `array[i+1]` calls take `%lsr.iv` directly;
the `array[i]` calls take `%3`):

| Line | Constant | Role |
|---|---|---|
| `%lsr.iv = phi i32 [ 1, … ]` | `1` | **Start**: the register's value
on loop entry (`i + 1`) (holds 1, 2, 3, …) |
| `%3 = add i32 %lsr.iv, -1` | `-1` | **Use-site offset**: derives the
*other* index (`i`) from the register each iteration. (No register is
dedicated to `%3`) |
| `%lsr.iv.next = add … , 1` | `+1` | **Step**: identical |

The two solutions are mirror images with exactly equal cost, because
`getSetupCost()` (the patched function) rated *any* constant induction
variable (IV) start equally (a 0-start and a 1-start IV cost the same).
LSR keeps the first solution it enumerates, and the enumeration order
derives from walking SSA use-lists, whose in-memory order differed
between our Linux/macOS and Windows builds of LLVM.

Thus both platforms compute the indices `i` and `i + 1` each iteration
(functionally identical), but the LSR decision of which of the two
recurrences becomes the loop-carried value (the phi) differs, which
affects register allocation.

> [!NOTE]
> The equal cost tie can only arise if LSR processes the loop at all,
and LSR only forms uses for induction variables of a 32-bit or 64-bit
integer in our case.

## Partial solution

The [upstream fix](https://github.com/llvm/llvm-project/pull/190587)
updates `getSetupCost()` to return `0` for constant IV starts whose
materialization is `TCC_Free` (which immediate `0` on RISC-V is), which
is now cheaper than non-zero starts, resulting in the same LSR decision
(no tie to decide on) on Windows as well.

As zero-start IVs are most common, this fix addresses the most common
scenarios, but it is still only a partial fix as it specifically solves
only the zero-start case. (I'm currently writing up another issue
regarding this.)

## Verification

I created a draft LLVM release that was built with the patch applied,
then reran all workflows using the draft release, including temporarily
testing more opt settings for the bytecode comparisons which ran
successfully (no mismatches) with the following `retester` modes (see
[this
run](https://github.com/paritytech/revive/actions/runs/28970814016/job/85982029937)):
* `NY M0 S+`
* `NY M0 S-`
* `NY M3 S+`
* `NY M3 S-`
* `NY Mz S+`
* `NY Mz S-`
* `Y M0 S+`
* `Y M0 S-`
* `Y M3 S+`
* `Y M3 S-`
* `Y Mz S+`
* `Y Mz S-`

All E2E tests also ran as it does today (allowing all Yul and newyork IR
`retester` modes), completing successfully.

2026-07-09 13:17:28 +02:00

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