interlocked-float-bit-cast-trick
Status: stub. The full-length analysis is queued for a v1.0.x patch release per ADR 0018, section 5, criterion #6. The companion rule page at docs/rules/interlocked-float-bit-cast-trick.md contains the canonical detection logic + GPU reasoning.
TL;DR
Atomic min/max on floats is needed in several common workloads: depth-of-field min-Z buffers, soft-shadow blocker depth reductions, voxel-cone tracing min-distance writes, and TAA history-clip range computations. Before SM 6.6, HLSL had no atomic float intrinsic, so shaders cast the float to uint, exploited the property that IEEE 754 single-precision floats compare correctly as signed magnitudes when sign-flipped, did the integer atomic, then cast back. The trick is correct, but it expands to roughly 8-12 instructions per atomic call site (sign extraction, conditional XOR, integer atomic, inverse sign restoration, asfloat), and requires the shader author to remember the sign convention for their value range — a frequent source of subtle off-by-sign bugs in negative-depth or negative-LOD code paths.
What the rule fires on
The hand-rolled idiom for atomic min/max on floating-point data: a sequence of asuint(x), conditional sign-bit flip (x ^ 0x80000000 for negatives, ~x for the inverted-comparison trick), InterlockedMin / InterlockedMax on the resulting uint, and a final asfloat to recover the value. The rule fires when the surrounding shader is compiled for SM 6.6 or higher, where InterlockedMin(float, float) and InterlockedMax(float, float) are native intrinsics and the bit-cast dance is no longer necessary.
See the What it detects section of the rule page for the full pattern definition.
Why it matters
The full GPU-mechanism analysis lives in the Why it matters on a GPU section of the companion rule page.
Examples
The bad / good code snippets are kept canonical on the rule page; see interlocked-float-bit-cast-trick.md -> Examples.
See also
- Rule page -- canonical reference + change log.
- workgroup overview -- broader context.
- ADR 0018 -- v1.0 readiness plan.
This is a v1.0-ship stub. Full analysis pending; track issue link TBD.