From ca0bdd238f29f2fd17ae44eec913bd6c82e0c1fe Mon Sep 17 00:00:00 2001
From: Jay Kwak <82421531+jkwak-work@users.noreply.github.com>
Date: Thu, 5 Jun 2025 10:08:22 -0700
Subject: Implement isnan and isinf for WGSL with bitwise operations (#7344)

WGSL doesn't support isnan and isinf, because it assumes that it always uses fast-math and fast-math doesnt' handle NaN as defined in IEEE standard.

The initial implementation used a clever workaround but it stopped working from some point.

This PR implemented isnan and isinf with a bitwise operation, which can be expensive.
But that seems to be an only option at the moment.
---
 source/slang/hlsl.meta.slang | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

(limited to 'source')

diff --git a/source/slang/hlsl.meta.slang b/source/slang/hlsl.meta.slang
index 6a3ae63ec..e00108e96 100644
--- a/source/slang/hlsl.meta.slang
+++ b/source/slang/hlsl.meta.slang
@@ -10628,7 +10628,15 @@ bool isinf(T x)
     case spirv:
         return spirv_asm { result:$$bool = OpIsInf $x};
     case wgsl:
-        __intrinsic_asm "($0 > 0x1.fffffep+127f) || ($0 < -0x1.fffffep+127f)";
+        static_assert(T is float, "isnan is implemented only for float type");
+        if (let f = x as float)
+        {
+            let bits = asuint(f);
+            let exp = (bits >> 23) & 0xffu;
+            let frac = bits & 0x7fffffu;
+            return exp == 0xffu && frac == 0u;
+        }
+        return false;
     }
 }
 
@@ -10684,7 +10692,15 @@ bool isnan(T x)
     case spirv:
         return spirv_asm { result:$$bool = OpIsNan $x};
     case wgsl:
-         __intrinsic_asm "$0 != $0";
+        static_assert(T is float, "isnan is implemented only for float type");
+        if (let f = x as float)
+        {
+            let bits = asuint(f);
+            let exp = (bits >> 23) & 0xffu;
+            let frac = bits & 0x7fffffu;
+            return exp == 0xffu && frac != 0u;
+        }
+        return false;
     }
 }
 
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