From: Chris Lattner Date: Mon, 7 Jan 2008 21:59:58 +0000 (+0000) Subject: add a note that is important for some fp apps. X-Git-Url: http://demsky.eecs.uci.edu/git/?a=commitdiff_plain;h=84a7c41251d7e647456176209110255326a7ebc0;p=oota-llvm.git add a note that is important for some fp apps. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@45723 91177308-0d34-0410-b5e6-96231b3b80d8 --- diff --git a/lib/Target/X86/README.txt b/lib/Target/X86/README.txt index 759c7acf389..be37acfa8a2 100644 --- a/lib/Target/X86/README.txt +++ b/lib/Target/X86/README.txt @@ -1597,3 +1597,24 @@ a stride-4 IV, would would allow all the scales in the loop to go away. This would result in smaller code and more efficient microops. //===---------------------------------------------------------------------===// + +In SSE mode, we turn abs and neg into a load from the constant pool plus a xor +or and instruction, for example: + + xorpd LCPI2_0-"L2$pb"(%esi), %xmm2 + +However, if xmm2 gets spilled, we end up with really ugly code like this: + + %xmm2 = reload [mem] + xorpd LCPI2_0-"L2$pb"(%esi), %xmm2 + store %xmm2 -> [mem] + +Since we 'know' that this is a 'neg', we can actually "fold" the spill into +the neg/abs instruction, turning it into an *integer* operation, like this: + + xorl 2147483648, [mem+4] ## 2147483648 = (1 << 31) + +you could also use xorb, but xorl is less likely to lead to a partial register +stall. + +//===---------------------------------------------------------------------===//