--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/clang/lib/CodeGen/README.txt	Thu Feb 13 15:10:13 2020 +0900
@@ -0,0 +1,47 @@
+IRgen optimization opportunities.
+
+//===---------------------------------------------------------------------===//
+
+The common pattern of
+--
+short x; // or char, etc
+(x == 10)
+--
+generates an zext/sext of x which can easily be avoided.
+
+//===---------------------------------------------------------------------===//
+
+Bitfields accesses can be shifted to simplify masking and sign
+extension. For example, if the bitfield width is 8 and it is
+appropriately aligned then is is a lot shorter to just load the char
+directly.
+
+//===---------------------------------------------------------------------===//
+
+It may be worth avoiding creation of alloca's for formal arguments
+for the common situation where the argument is never written to or has
+its address taken. The idea would be to begin generating code by using
+the argument directly and if its address is taken or it is stored to
+then generate the alloca and patch up the existing code.
+
+In theory, the same optimization could be a win for block local
+variables as long as the declaration dominates all statements in the
+block.
+
+NOTE: The main case we care about this for is for -O0 -g compile time
+performance, and in that scenario we will need to emit the alloca
+anyway currently to emit proper debug info. So this is blocked by
+being able to emit debug information which refers to an LLVM
+temporary, not an alloca.
+
+//===---------------------------------------------------------------------===//
+
+We should try and avoid generating basic blocks which only contain
+jumps. At -O0, this penalizes us all the way from IRgen (malloc &
+instruction overhead), all the way down through code generation and
+assembly time.
+
+On 176.gcc:expr.ll, it looks like over 12% of basic blocks are just
+direct branches!
+
+//===---------------------------------------------------------------------===//