[llvm] [AMDGPU] Avoid hexdigit assert in emitBaseName (PR #219034)

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<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-tablegen

@llvm/pr-subscribers-backend-amdgpu

Author: Jacob Lambert (lamb-j)

<details>
<summary>Changes</summary>

`hexdigit()` asserts above `0xF`, so a `SubArchSpelling` variant with a wider
stepping crashes TableGen. Name the low nibble.

No in-tree target has such a stepping, so the generated tables are unchanged.


---
Full diff: https://github.com/llvm/llvm-project/pull/219034.diff


2 Files Affected:

- (modified) llvm/test/TableGen/AMDGPUTargetDefSubArchSpelling.td (+10) 
- (modified) llvm/utils/TableGen/Basic/AMDGPUTargetDefEmitter.cpp (+3-2) 


``````````diff
diff --git a/llvm/test/TableGen/AMDGPUTargetDefSubArchSpelling.td b/llvm/test/TableGen/AMDGPUTargetDefSubArchSpelling.td
index 8d06a3264112c..b5b292ce49fc7 100644
--- a/llvm/test/TableGen/AMDGPUTargetDefSubArchSpelling.td
+++ b/llvm/test/TableGen/AMDGPUTargetDefSubArchSpelling.td
@@ -22,6 +22,13 @@ def GFX888_AMAZING : ProcessorModel<"gfx888-amazing", NoSchedModel, []>,
   let SubArchSpelling = "8.88a";
 }
 
+// A variant whose stepping is wider than a hex digit: only the low nibble is
+// named.
+def GFX888_WIDE : ProcessorModel<"gfx888-wide", NoSchedModel, []>,
+    AMDGPUGPUInfo<[8, 8, 0xfff8]> {
+  let SubArchSpelling = "8.88w";
+}
+
 // The variant contributes a distinct GPUKind.
 // CHECK: GK_GFX888
 // CHECK: GK_GFX888_AMAZING
@@ -33,12 +40,15 @@ def GFX888_AMAZING : ProcessorModel<"gfx888-amazing", NoSchedModel, []>,
 // CHECK: "gfx888\0"
 // CHECK: "gfx8\0"
 // CHECK: "gfx888-amazing\0"
+// CHECK: "gfx888-wide\0"
 // CHECK: "amdgpu8.88a\0"
+// CHECK: "amdgpu8.88w\0"
 
 // The GPU table: the base GPU has no base name (offset 0); the variant uses
 // AMDGPUSubArch888A and records "gfx888" as its base name.
 // CHECK: {[[#]], Triple::AMDGPUSubArch888, {{.*}}, {8, 8, 8}, [[FAM:[0-9]+]], 0},
 // CHECK: {[[#]], Triple::AMDGPUSubArch888A, {{.*}}, {8, 8, 8}, [[FAM]], [[#]]},
+// CHECK: {[[#]], Triple::AMDGPUSubArch888W, {{.*}}, {8, 8, 65528}, [[FAM]], [[#]]},
 
 // The subarch-name table maps the variant's own subarch to its triple name.
 // CHECK: {Triple::AMDGPUSubArch888A, [[#]], [[#]]},
diff --git a/llvm/utils/TableGen/Basic/AMDGPUTargetDefEmitter.cpp b/llvm/utils/TableGen/Basic/AMDGPUTargetDefEmitter.cpp
index b1d1afd846715..a35e7bfe72496 100644
--- a/llvm/utils/TableGen/Basic/AMDGPUTargetDefEmitter.cpp
+++ b/llvm/utils/TableGen/Basic/AMDGPUTargetDefEmitter.cpp
@@ -112,12 +112,13 @@ static void emitArchFamily(raw_ostream &OS, const Record *Rec) {
   OS << "gfx" << Rec->getValueAsListOfInts("IsaVersion")[0];
 }
 
-// Emit the canonical GPU name for a variant (empty for a non-variant GPU).
+// Emit the canonical GPU name for a variant (empty for a non-variant GPU). Only
+// the stepping's low nibble is named, so a wider stepping does not assert.
 static void emitBaseName(raw_ostream &OS, const Record *Rec) {
   if (!getSubArchSpelling(Rec))
     return;
   std::vector<int64_t> V = Rec->getValueAsListOfInts("IsaVersion");
-  OS << "gfx" << V[0] << V[1] << hexdigit(V[2], /*LowerCase=*/true);
+  OS << "gfx" << V[0] << V[1] << hexdigit(V[2] & 0xF, /*LowerCase=*/true);
 }
 
 // Emit the ISA version tuple as "major, minor, stepping" wrapped in \p Open and

``````````

</details>


https://github.com/llvm/llvm-project/pull/219034