gfx-rs/wgpu · error

Unexpected min({:?})

Error message

Unexpected min({:?})

What it means

The SPIR-V backend lowers MathFunction::Min through GlslStd450 FMin/SMin/UMin based on arg_scalar_kind; a Float/Sint/Uint argument is fine, but any other kind (None, Bool, etc.) reaches `unimplemented!("Unexpected min({:?})", other)`. The panic occurs in cache_expression_value invoked by write_block, halting shader translation.

Source

Thrown at naga/src/back/spv/block.rs:1406

                                MathOp::Ext(spirv::GlslStd450Op::FAbs)
                            }
                            Some(crate::ScalarKind::Sint) => MathOp::Ext(spirv::GlslStd450Op::SAbs),
                            Some(crate::ScalarKind::Uint) => {
                                MathOp::Custom(Instruction::unary(
                                    spirv::Op::CopyObject, // do nothing
                                    result_type_id,
                                    id,
                                    arg0_id,
                                ))
                            }
                            other => unimplemented!("Unexpected abs({:?})", other),
                        }
                    }
                    Mf::Min => MathOp::Ext(match arg_scalar_kind {
                        Some(crate::ScalarKind::Float) => spirv::GlslStd450Op::FMin,
                        Some(crate::ScalarKind::Sint) => spirv::GlslStd450Op::SMin,
                        Some(crate::ScalarKind::Uint) => spirv::GlslStd450Op::UMin,
                        other => unimplemented!("Unexpected min({:?})", other),
                    }),
                    Mf::Max => MathOp::Ext(match arg_scalar_kind {
                        Some(crate::ScalarKind::Float) => spirv::GlslStd450Op::FMax,
                        Some(crate::ScalarKind::Sint) => spirv::GlslStd450Op::SMax,
                        Some(crate::ScalarKind::Uint) => spirv::GlslStd450Op::UMax,
                        other => unimplemented!("Unexpected max({:?})", other),
                    }),
                    Mf::Clamp => match arg_scalar_kind {
                        // Clamp is undefined if min > max. In practice this means it can use a median-of-three
                        // instruction to determine the value. This is fine according to the WGSL spec for float
                        // clamp, but integer clamp _must_ use min-max. As such we write out min/max.
                        Some(crate::ScalarKind::Float) => MathOp::Ext(spirv::GlslStd450Op::FClamp),
                        Some(_) => {
                            let (min_op, max_op) = match arg_scalar_kind {
                                Some(crate::ScalarKind::Sint) => {
                                    (spirv::GlslStd450Op::SMin, spirv::GlslStd450Op::SMax)
                                }
                                Some(crate::ScalarKind::Uint) => {

View on GitHub (pinned to 3e11ff59bf)

Solutions

1. Use min only on numeric scalar/vector arguments in the shader
2. Run naga validation before the spv backend to catch the bad call early
3. Inspect the IR dump to locate the offending min expression
4. Report a reproducer upstream if valid input panics

Example fix

// before
let m = min(a, b); // a,b: bool
// after
let m = min(a.x, b.x); // f32 components

assert!(matches!(arg_scalar_kind, Some(crate::ScalarKind::Float | crate::ScalarKind::Sint | crate::ScalarKind::Uint)), "min() requires numeric argument");

fn min_max_supported(kind: Option<crate::ScalarKind>) -> bool {
    matches!(kind, Some(crate::ScalarKind::Float | crate::ScalarKind::Sint | crate::ScalarKind::Uint))
}

When it happens

Trigger: A `min(a, b)` built-in whose argument scalar kind is not Float, Sint, or Uint — e.g. min over bools or a kindless/composite type — while generating SPIR-V.

Common situations: Direct backend use on unvalidated/fuzzed IR; front-end regression letting invalid min() through; WGSL sources with min on booleans that should have failed validation.

Related errors

• Unexpected abs({:?})
• Unexpected max({:?})
• not implemented
• metal does not support vertex ray hit return
• not implemented

AI-assisted analysis of gfx-rs/wgpu@3e11ff59bf (2026-09-03). Data as JSON: /api/errors/83e06051bacda542. Report an issue: GitHub.