metal: use 256-byte uniform alignment on iOS simulator#636
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The simulator on Apple Silicon Macs (`aarch64-apple-ios-sim`,
`target_abi = "sim"`) runs the iOS arm64 binary but issues Metal
commands against the host Mac's GPU subsystem. That subsystem
validates with the 256-byte rule — not the 16-byte rule for Apple
GPUs on real iOS devices. The existing cfg matched the simulator
into the `target_arch = "aarch64"` branch with the iOS device
constant, and the first draw asserted:
-[MTLDebugRenderCommandEncoder validateCommonDrawErrors:]:6032:
failed assertion `Draw Errors Validation
Vertex Function(vertexShader): the offset into the buffer
uniforms that is bound at Buffer index 0 must be a multiple
of 256 but was set to 144.`
Add `target_abi = "sim"` to the cfg group that selects the 256-byte
constant so both the legacy x86_64 simulator and the modern arm64
simulator pick the same alignment as macOS. Real arm64 iOS devices
keep the 16-byte alignment they correctly used before.
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Problem
The iOS simulator on Apple Silicon Macs (
aarch64-apple-ios-sim,target_abi = "sim") runs the iOS arm64 binary but issues Metal commands against the host Mac's GPU subsystem. That subsystem validates with the 256-byte uniform buffer offset rule — not the 16-byte rule for Apple GPUs on real iOS devices.The existing cfg matched the simulator into the
target_arch = "aarch64"branch with the iOS device's 16-byte constant. The first draw asserts:Fix
Add
target_abi = "sim"to the cfg group that selects the 256-byte constant. Both the legacy x86_64 simulator and the modern arm64 simulator now pick the same alignment as macOS. Real arm64 iOS devices keep the 16-byte alignment they correctly used before.Tested on
iPhone 17 simulator on iOS 27 beta (Apple Silicon Mac host) — uniform-alignment assert gone, draws complete.