[llvm] [SPIR-V] Implement SPV_KHR_float_controls2 (PR #146941)

[Nathan Gauër via llvm-commits]

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@@ -1978,8 +2072,43 @@ static unsigned getFastMathFlags(const MachineInstr &I) {
     Flags |= SPIRV::FPFastMathMode::NSZ;
   if (I.getFlag(MachineInstr::MIFlag::FmArcp))
     Flags |= SPIRV::FPFastMathMode::AllowRecip;
-  if (I.getFlag(MachineInstr::MIFlag::FmReassoc))
-    Flags |= SPIRV::FPFastMathMode::Fast;
+  if (I.getFlag(MachineInstr::MIFlag::FmContract) && CanUseKHRFloatControls2)
+    Flags |= SPIRV::FPFastMathMode::AllowContract;
+  if (I.getFlag(MachineInstr::MIFlag::FmReassoc)) {
+    if (CanUseKHRFloatControls2)
+      // LLVM reassoc maps to SPIRV transform, see
+      // https://github.com/KhronosGroup/SPIRV-Registry/issues/326 for details.
+      // Because we are enabling AllowTransform, we must enable AllowReassoc and
+      // AllowContract too, as required by SPIRV spec. Also, we used to map
+      // MIFlag::FmReassoc to FPFastMathMode::Fast, which now should instead by
+      // replaced by turning all the other bits instead. Therefore, we're
+      // enabling every bit here except None and Fast.
+      Flags |= SPIRV::FPFastMathMode::NotNaN | SPIRV::FPFastMathMode::NotInf |
+               SPIRV::FPFastMathMode::NSZ | SPIRV::FPFastMathMode::AllowRecip |
+               SPIRV::FPFastMathMode::AllowTransform |
+               SPIRV::FPFastMathMode::AllowReassoc |
+               SPIRV::FPFastMathMode::AllowContract;
+    else
+      Flags |= SPIRV::FPFastMathMode::Fast;
+  }
+
+  if (CanUseKHRFloatControls2) {
+    // Error out if SPIRV::FPFastMathMode::Fast is enabled.
+    if (Flags & SPIRV::FPFastMathMode::Fast)
+      report_fatal_error(
+          "FPFastMathMode::Fast flag is deprecated and it is not "
+          "valid to use anymore.");
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Keenuts wrote:

Looks like this is not a runtime error: only an error in this function would result in `Fast` being set with `CanUseKHRFloatControls2`, so assert would be more appropriate.

https://github.com/llvm/llvm-project/pull/146941