<html> <head> <base href="https://bugs.freedesktop.org/" /> </head> <body> <div> <a class="bz_bug_link bz_status_NEW " title="NEW --- - clamp with bounds inside [0, 1] generates slow code" href="https://bugs.freedesktop.org/show_bug.cgi?id=76862#c2">Comment # 2</a> on <a class="bz_bug_link bz_status_NEW " title="NEW --- - clamp with bounds inside [0, 1] generates slow code" href="https://bugs.freedesktop.org/show_bug.cgi?id=76862">bug 76862</a> from <a class="email" href="mailto:mattst88@gmail.com" title="Matt Turner <mattst88@gmail.com>"> Matt Turner</a> <pre>Ian's description has bugs, so let me try again: GLSL's clamp(A, B, C) clamps A to a lower bound of B and an upper bound of C. We implement this in the compiler with min() and max() operations: min(max(a, b), c). i965 assembly for clamp(A, B, C) would look like (select from A, B the argument that is greater than or equal; i.e., max) sel.ge tmp, A, B (select from tmp, C the argument that is less that; i.e., min) sel.l dst, tmp, C Saturate is a special case of clamp, specifically when the bounds are 0.0 to 1.0 (for floating point types). Probably all GPUs can perform saturate for free -- it's a destination modifier in i965 assembly. The i965 backend's try_emit_saturate() function recognizes min(max(a, 0.0), 1.0) as a saturate operation, and sets the saturate modifier (or emits a MOV instruction with saturate). i965 assembly for clamp(A, 0.0, 1.0) (after try_emit_saturate()) would turn into mov.sat dst, A The proposed optimization idea here is that for immediate arguments that satisfy the condition in <a href="show_bug.cgi?id=76862#c0">comment #0</a>, we can emit a single min/max instruction with a saturate modifier instead of a min and a max instruction. So, clamp(A, 0, 0.5) would be (select the least of A and 0.5, saturate the result, and store in dst) sel.l.sat dst, A, 0.5 and similarly clamp(A, 0.1, 1) -> sel.ge.sat dst, A, 0.1.</pre> </div> <hr> You are receiving this mail because: <ul> <li>You are the QA Contact for the bug.</li> </ul> </body> </html>