From b0daffc609f4e42fe81d4da641f7a9dc947bad53 Mon Sep 17 00:00:00 2001 From: Chris Lattner Date: Sun, 16 Jan 2011 06:39:44 +0000 Subject: [PATCH] add some commentary git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@123572 91177308-0d34-0410-b5e6-96231b3b80d8 --- lib/Target/README.txt | 15 ++++++++++++++- 1 file changed, 14 insertions(+), 1 deletion(-) diff --git a/lib/Target/README.txt b/lib/Target/README.txt index 8039790606c..6ba0d745f61 100644 --- a/lib/Target/README.txt +++ b/lib/Target/README.txt @@ -2155,8 +2155,12 @@ _ZNSt12_Vector_baseIiSaIiEEC2EmRKS0_.exit.i.i: ; preds = %cond.true.i.i.i.i 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 @@ This shouldn't need the ((zext (%n - 1)) + 1) game, and it should ideally fold 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 @@ S is only 6 bytes, but each element is 8 byte-aligned. We generate a loop and 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: -- 2.34.1