How do you specify the [[loop]] attribute in shaders?

[Qix-]

I have a pretty hefty loop that the compiler is trying to unroll for me, and I'd like to tell it not to.

Error: D3DCompile failed 0x80004005 (89,8-60): warning X3570: gradient instruction used in a loop with varying iteration, attempting to unroll the loop
(331,1-33): error X3511: unable to unroll loop, loop does not appear to terminate in a timely manner (680 iterations) or unrolled loop is too large, use the [unroll(n)] attribute to force an exact higher number

However adding either [[loop]] or simply [loop] before the for causes shaderc to bark about syntax:

	[[loop]] for (i = 0; i < MAX_STEPS; i += 1.0) {

Error: D3DCompile failed 0x80004005 (331,2): error X3000: syntax error: unexpected token '['
(331,42): error X3000: syntax error: unexpected token ')'

How can I tell the compiler not to unroll the loop?

Even setting MAX_STEPS to a lower number results in long compilation times and large shaders, so I'm pretty sure it's still trying to unroll at every chance. Is unrolling a loop a requirement?

[Qix-]

Ah okay! It's just GLSL that was having issues - at least, on all windows-supported shader variants. It might be required that special handling for Metal is also needed, but I'm not on a Mac at the moment to test.

#if BGFX_SHADER_LANGUAGE_GLSL
#	pragma unroll 1
#else
	[loop]
#endif
	for (...) {

Works great!

[bkaradzic]

See:

bgfx/src/bgfx_shader.sh

Line 17 in 9204a53

# define LOOP [loop]

[pezcode]

bgfx_shader.sh defines BRANCH, LOOP, and UNROLL that you can use:

bgfx/src/bgfx_shader.sh

Line 15 in 9204a53

#if BGFX_SHADER_LANGUAGE_HLSL > 300

But they're no-ops on anything but HLSL. If you need that GLSL pragma you'll still need to roll your own (or send a pull request that adds it for GLSL 🤠)

[Qix-]

Ohoho thank you! I think on GLSL in order to prevent it from trying to unroll I still need it, and I don't know how you'd do that as a define for GLSL anyway since you can't nest pre-processor directives.