[llvm] [GlobalISel] Optimize ULEB128 usage (PR #90565)

[Jay Foad via llvm-commits]

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@@ -713,6 +713,27 @@ class GIMatchTableExecutor {
     memcpy(&Ret, MatchTable, sizeof(Ret));
     return Ret;
   }
+
+public:
+  // Faster ULEB128 decoder tailored for the Match Table Executor.
+  //
+  // - Arguments are fixed to avoid mid-function checks.
+  // - Unchecked execution, assumes no error.
+  // - Fast common case handling (1 byte values).
+  LLVM_ATTRIBUTE_ALWAYS_INLINE static uint64_t
+  fastDecodeULEB128(const uint8_t *__restrict MatchTable,
+                    uint64_t &CurrentIdx) {
+    uint64_t Value = MatchTable[CurrentIdx] & 0x7f;
+    if (LLVM_UNLIKELY(MatchTable[CurrentIdx++] >= 128)) {
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jayfoad wrote:

Move the `& 0x7f` inside the `if`?

https://github.com/llvm/llvm-project/pull/90565