| Issue |
183747
|
| Summary |
AArch64: 'Unsupported library call operation' for uitofp/fptosi i256 <-> double
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
koko1123
|
## Description
The AArch64 backend crashes with `LLVM ERROR: Unsupported library call operation!` when lowering `uitofp i256 %val to double` or `fptoui double %val to i256`. This works on x86_64 but fails on AArch64.
This is a regression -- Zig 0.15.2 (which bundles LLVM 19) can compile this in some contexts, but Zig 0.16.0-dev (LLVM 21.1) crashes consistently.
## Reproducer (LLVM IR)
```llvm
define double @u256_to_f64(i256 %val) {
%result = uitofp i256 %val to double
ret double %result
}
define i256 @f64_to_u256(double %val) {
%result = fptoui double %val to i256
ret i256 %result
}
```
```bash
# Save as repro.ll and run:
llc -mtriple=aarch64 repro.ll -o /dev/null
# LLVM ERROR: Unsupported library call operation!
```
## Reproducer (Zig, which emits the above IR)
```zig
const std = @import("std");
const ETHER: u256 = 1_000_000_000_000_000_000;
fn parseEther(ether: f64) u256 {
const wei_f = ether * @as(f64, @floatFromInt(ETHER));
return @intFromFloat(wei_f);
}
test "parse ether" {
const x = parseEther(1.0);
try std.testing.expect(x > 0);
}
```
```bash
zig test repro.zig # on aarch64-macos
# LLVM ERROR: Unsupported library call operation!
```
## Context
This affects real-world code in Ethereum libraries that use u256 (the native word size for the EVM) with float conversions for unit formatting (wei <-> ether). The workaround is to route through u128 intermediates, but the x86_64 backend handles i256 float conversions correctly so this appears to be an AArch64-specific gap.
## Related issues
- [ziglang/zig#18820](https://github.com/ziglang/zig/issues/18820) -- Zig tracker for this LLVM limitation
- [StrobeLabs/eth.zig#9](https://github.com/StrobeLabs/eth.zig/issues/9) -- Real-world impact on Ethereum library
- #91979 -- Similar "Unsupported library call operation" on AArch64 (closed as x86_fp80-specific, but the underlying lowering gap is related)
- #52675 -- Similar crash with _ExtInt types
## Environment
- LLVM 21.1 (bundled in Zig 0.16.0-dev.2565+684032671)
- Target: aarch64-macos (Apple Silicon M-series)
- Also affects: aarch64-linux (per ziglang/zig#18820)
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