[llvm] [NVPTX] Respect FTZ flag when lowering atomicrmw fadd. (PR #200732)

[Justin Lebar via llvm-commits]

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@@ -108,6 +108,16 @@ static cl::opt<bool> UsePrecSqrtF32(
     cl::desc("NVPTX Specific: 0 use sqrt.approx, 1 use sqrt.rn."),
     cl::init(true));
 
+// PTX atom.add.f32 has fixed FTZ behavior that may not match the function's
+// (see shouldExpandAtomicRMWInIR), so by default we fall back to a CAS loop
+// when they disagree. This flag is an escape hatch to use atom.add anyway,
+// trading correct denormal handling for the speed of the native instruction.
+static cl::opt<bool> AllowFTZAtomics(
+    "nvptx-allow-ftz-atomics", cl::Hidden,
+    cl::desc("NVPTX Specific: Lower atomicrmw fadd to atom.add even when its "
+             "FTZ behavior does not match the function's denormal mode."),
+    cl::init(false));
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jlebar wrote:

> if a frontend wants some of the atomics it generates to use FTZ, that frontend just sets FTZ for those atomics.  What are the downsides of this approach?

I don't believe LLVM IR allows you to set FTZ on individual instructions.  So I would say "this is not possible with LLVM IR as it exists today" is the downside to the approach.  :)

https://github.com/llvm/llvm-project/pull/200732