diff --git a/llvm/docs/GlobalISel.rst b/llvm/docs/GlobalISel.rst
--- a/llvm/docs/GlobalISel.rst
+++ b/llvm/docs/GlobalISel.rst
@@ -396,6 +396,90 @@
 should be (you could imagine disagreements on whether a ``<2 x s8>``
 operation should be scalarized or extended to ``<8 x s8>``).
 
+.. _min-legalizerinfo:
+
+Minimum Rule Set
+^^^^^^^^^^^^^^^^
+
+GlobalISel's legalizer has a great deal of flexibility in how a given target
+shapes the gMIR that the rest of the backend must handle. However, there are
+a small number of requirements that all targets must meet.
+
+Before discussing the minimum requirements, we'll need some terminology.
+Each target has a set of types which is the union of all possible types produced
+by at least one legal instruction. We'll call this the Producer Type Set.
+Similarly, each target has a set of types which is the union of all possible
+types consumed by at least one legal instruction. We'll call this the Consumer
+Type Set. Both sets are often identical but there's no guarantee of that. For
+example, it's not uncommon to be unable to consume s64 but still be able to
+produce it for a few specific instructions.
+
+Minimum Rules For Scalars
+"""""""""""""""""""""""""
+
+There are three operations produced by the IRTranslator that can increase the
+size of a scalar:
+* G_ANYEXT
+* G_SEXT
+* G_ZEXT
+* G_FPEXT
+of these, the only one with mandatory legality is G_ANYEXT. It's required that
+G_ANYEXT is legal for all inputs from the producer type set and all larger
+outputs from the consumer type set. The lack of any defined value for the
+additional bits usually makes this trivial and this ensures that we can always
+lower other type conversions using G_ANYEXT and some operations on the larger
+value. For example, G_SEXT can always lower to:
+  %1 = G_ANYEXT %0
+  %2 = G_CONSTANT ...
+  %3 = G_SHL %1, %2
+  %4 = G_ASHR %3, %2
+and the G_CONSTANT/G_SHL/G_ASHR can further lower to other operations or target
+instructions. Similarly, G_FPEXT has no legality requirement since it can lower
+to a G_ANYEXT followed by a target instruction.
+
+G_TRUNC has the same requirements as G_ANYEXT but with the input/output types
+reversed.
+
+G_MERGE_VALUES and G_UNMERGE_VALUES do not have legality requirements since the
+former can lower to G_ANYEXT and some other legalizable instructions, while the
+latter can lower to some legalizable instructions followed by G_TRUNC.
+
+Minimum Legality For Vectors
+""""""""""""""""""""""""""""
+
+Within the vector types, there aren't any defined conversions in LLVM-IR as
+vectors are often converted by reinterpreting the bits or by decomposing the
+vector and reconstituting it as a different type. As such, G_BITCAST is the
+only operation to account for. We don't require that it's legal because it
+can be lowered to COPY (or to nothing using replaceAllUses()).
+
+There are no legality requirements for G_BUILD_VECTOR, or G_BUILD_VECTOR_TRUNC
+since these can be lowered to target instructions which are legal by definition.
+The same reasoning also allows G_UNMERGE_VALUES to lack legality requirements
+for vector inputs.
+
+Minimum Legality for Pointers
+"""""""""""""""""""""""""""""
+
+There are no minimum rules for pointers since G_INTTOPTR and G_PTRTOINT can
+be lowered to COPY (or to nothing using replaceAllUses()) by the legalizer.
+
+Minimum Legality For Operations
+"""""""""""""""""""""""""""""""
+
+The rules for G_ANYEXT, G_MERGE_VALUES, G_BITCAST, G_BUILD_VECTOR,
+G_BUILD_VECTOR_TRUNC, G_CONCAT_VECTORS, G_UNMERGE_VALUES, G_PTRTOINT, and
+G_INTTOPTR have already been noted above. In addition to those, the following
+operations have requirements:
+* At least one G_IMPLICIT_DEF must be legal. This is usually trivial as it
+  requires no code to be selected.
+* G_PHI must be legal for all types in the producer and consumer typesets. This
+  is usually trivial as it requires no code to be selected.
+* At least one G_FRAME_INDEX must be legal
+* At least one G_BLOCK_ADDR must be legal
+
+There are many other operations you'd expect to have legality requirements but
+they can be lowered to target instructions which are legal by definition.
 
 .. _regbankselect: