[GlobalIsel] Add Gallery to MIR Patterns (llvm#89974)

examples for fold of zext(trunc:nuw)

Author: tschuett

llvm/docs/GlobalISel/MIRPatterns.rst

@@ -514,3 +514,40 @@ of operands.
     (match (does_not_bind $tmp, $x)
            (G_MUL $dst, $x, $tmp)),
     (apply (COPY $dst, $x))>;
+
+
+
+
+Gallery
+=======
+
+We should use precise patterns that state our intentions. Please avoid
+using wip_match_opcode in patterns.
+
+.. code-block:: text
+  :caption: Example fold zext(trunc:nuw)
+
+  // Imprecise: matches any G_ZEXT
+  def zext : GICombineRule<
+    (defs root:$root),
+    (match (wip_match_opcode G_ZEXT):$root,
+    [{ return Helper.matchZextOfTrunc(*${root}, ${matchinfo}); }]),
+    (apply [{ Helper.applyBuildFn(*${root}, ${matchinfo}); }])>;
+
+
+  // Imprecise: matches G_ZEXT of G_TRUNC
+  def zext_of_trunc : GICombineRule<
+    (defs root:$root),
+    (match (G_TRUNC $src, $x),
+           (G_ZEXT $root, $src),
+    [{ return Helper.matchZextOfTrunc(${root}, ${matchinfo}); }]),
+    (apply [{ Helper.applyBuildFnMO(${root}, ${matchinfo}); }])>;
+
+
+  // Precise: matches G_ZEXT of G_TRUNC with nuw flag
+  def zext_of_trunc_nuw : GICombineRule<
+    (defs root:$root),
+    (match (G_TRUNC $src, $x, (MIFlags NoUWrap)),
+           (G_ZEXT $root, $src),
+    [{ return Helper.matchZextOfTrunc(${root}, ${matchinfo}); }]),
+    (apply [{ Helper.applyBuildFnMO(${root}, ${matchinfo}); }])>;