[llvm-commits] [llvm] r123572 - /llvm/trunk/lib/Target/README.txt

[Chris Lattner]

Author: lattner
Date: Sun Jan 16 00:39:44 2011
New Revision: 123572

URL: http://llvm.org/viewvc/llvm-project?rev=123572&view=rev
Log:
add some commentary

Modified:
    llvm/trunk/lib/Target/README.txt

Modified: llvm/trunk/lib/Target/README.txt
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/README.txt?rev=123572&r1=123571&r2=123572&view=diff
==============================================================================
--- llvm/trunk/lib/Target/README.txt (original)
+++ llvm/trunk/lib/Target/README.txt Sun Jan 16 00:39:44 2011
@@ -2155,8 +2155,12 @@
   br label %_ZNSt6vectorIiSaIiEEC1EmRKiRKS0_.exit
 
 This is just the handling the construction of the vector. Most surprising here
-is the fact that all three null stores in %entry are dead, but not eliminated.
+is the fact that all three null stores in %entry are dead (because we do no
+cross-block DSE).
+
 Also surprising is that %conv isn't simplified to 0 in %....exit.thread.i.i.
+This is a because the client of LazyValueInfo doesn't simplify all instruction
+operands, just selected ones.
 
 //===---------------------------------------------------------------------===//
 
@@ -2192,6 +2196,9 @@
 the two memset's together. The issue with %n seems to stem from poor handling
 of the original loop.
 
+To simplify this, we need SCEV to know that "n != 0" because of the dominating
+conditional.  That would turn the second memset into a simple memset of 'n'.
+
 //===---------------------------------------------------------------------===//
 
 clang -O3 -fno-exceptions currently compiles this code:
@@ -2211,6 +2218,12 @@
 4 stores on each iteration. If the struct were 8 bytes, this gets turned into
 a memset.
 
+In order to handle this we have to:
+  A) Teach clang to generate metadata for memsets of structs that have holes in
+     them.
+  B) Teach clang to use such a memset for zero init of this struct (since it has
+     a hole), instead of doing elementwise zeroing.
+
 //===---------------------------------------------------------------------===//
 
 clang -O3 currently compiles this code: