Updated documentation based on the recent review comments

Author: rajatbajpai

llvm/lib/Target/NVPTX/NVPTX.td

@@ -33,43 +33,48 @@ class FeaturePTX<int version>:
    SubtargetFeature<"ptx"# version, "PTXVersion",
                     "" # version,
                     "Use PTX version " # version>;
-//
 // NVPTX Architecture Hierarchy and Ordering:
-// 
-// Family: 2/3/5/6/7/8/9/10/12 (Follows Onion model, older family is compatible with newer family)
-// Arch: 2*/3*/5*/6*/7*/8*/9*/10*/12*
 //
-// Family-specific: F*f : F*f > F* =>
-// + The plain base architecture is compatible with the family-specific architecture
-//   (e.g. sm_100 compatible with >= sm_100*f*)
-// + The family-specific architecture is compatible with future family-specific
-//   architectures within the same family (e.g. sm_100f compatible with >= sm_10X*f*
-//   but not with sm_12X*f*)
+// GPU architectures: sm_2Y/sm_3Y/sm_5Y/sm_6Y/sm_7Y/sm_8Y/sm_9Y/sm_10Y/sm_12Y
+// ('Y' represents version within the architecture)
+// The architectures have name of form sm_XYz where 'X' represent the generation
+// number, 'Y' represents the version within the architecture, and 'z' represents
+// the optional feature suffix.
+// If X1Y1 <= X2Y2, then GPU capabilities of sm_X1Y1 are included in sm_X2Y2.
+// For example, take sm_90 (9 represents 'X', 0 represents 'Y', and no feature
+// suffix) and sm_103 architectures (10 represents 'X', 3 represents 'Y', and no
+// feature suffix). Since 90 <= 103, sm_90 is compatible with sm_103.
 //
-//    Family and SM Target Definition:
-//    +----------------+--------------------------------------------------------+
-//    | Family         | Target SM architectures included                       |
-//    +----------------+--------------------------------------------------------+
-//    | sm_10x family  | sm_100f, sm_103f, future targets in sm_10x family      |
-//    | sm_101 family  | sm_101f (exception)                                    |
-//    | sm_12x family  | sm_120f, sm_121f, future targets in sm_12x family      |
-//    +----------------+--------------------------------------------------------+
+// The family-specific architectures have 'f' feature suffix and they follow
+// following order:
+// sm_X{Y2}f > sm_X{Y1}f iff Y2 > Y1
+// sm_XY{f} > sm_{XY}{}
 //
-// Architecture-specific: F*a : F*a > F*f > F* =>
-// + The plain base architecture is compatible with the architecture-specific architecture
-//   (e.g. sm_100 compatible with >= sm_100*a*)
-// + The family-specific architecture is compatible with the architecture-specific architecture
-//   (e.g. sm_100f compatible with >= sm_100*a*)
-// + The architecture-specific architecture is incompatible with any other architecture
-//   (e.g. sm_100a is only compatible with sm_100*a*)
+// For example, take sm_100f (10 represents 'X', 0 represents 'Y', and 'f'
+// represents 'z') and sm_103f (10 represents 'X', 3 represents 'Y', and 'f'
+// represents 'z') architectures. Since Y1 < Y2, sm_100f is compatible with
+// sm_103f. Similarly based on the second rule, sm_90 is compatible with sm_103f.
 //
-// Encoding: Arch * 1000 + 'f' * 10 + 'a' * 1 (where 'a' ⇒ 'f')
-// 
-// This encoding allows simple implementation of the partial ordering of the architectures.
-//  + Compare Family and Arch by dividing FullSMVersion by 1000 and 100 respectively before the comparison.
-//  + Compare within the family by comparing FullSMVersion, given both belongs to the same family.
-//  + Detect 'a' variants by checking FullSMVersion % 10.
+// The architecture-specific architectures have 'a' feature suffix and they follow
+// following order:
+// sm_XY{a} > sm_XY{f} > sm_{XY}{}
+//
+// For example, take sm_103a (10 represents 'X', 3 represents 'Y', and 'a'
+// represents 'z'), sm_103f, and sm_103 architectures. The sm_103 is compatible
+// with sm_103a and sm_103f, and sm_103f is compatible with sm_103a.
 //
+// Encoding := Arch * 100 + 10 (for 'f') + 1 (for 'a')
+// Arch := X * 10 + Y
+//
+// For example, sm_103a is encoded as 10311 (103 * 100 + 10 + 1) and sm_103f is
+// encoded as 10310 (103 * 100 + 10).
+//
+// This encoding allows simple partial ordering of the architectures.
+//  + Compare Family and Arch by dividing FullSMVersion by 1000 and 100
+//    respectively before the comparison.
+//  + Compare within the family by comparing FullSMVersion, given both belongs to
+//    the same family.
+//  + Detect 'a' variants by checking FullSMVersion % 10.
 foreach sm = [20, 21, 30, 32, 35, 37, 50, 52, 53,
               60, 61, 62, 70, 72, 75, 80, 86, 87,
               89, 90, 100, 101, 103, 120, 121] in {