[llvm-commits] [llvm] r121849 - /llvm/trunk/lib/Target/README.txt

[Chris Lattner]

Author: lattner
Date: Wed Dec 15 00:38:24 2010
New Revision: 121849

URL: http://llvm.org/viewvc/llvm-project?rev=121849&view=rev
Log:
add a note about a SPEC hack that gcc mainline does.

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=121849&r1=121848&r2=121849&view=diff
==============================================================================
--- llvm/trunk/lib/Target/README.txt (original)
+++ llvm/trunk/lib/Target/README.txt Wed Dec 15 00:38:24 2010
@@ -1226,6 +1226,29 @@
 
 actually a conditional increment: loadpre18.c loadpre19.c
 
+//===---------------------------------------------------------------------===//
+
+[LOAD PRE / STORE SINKING / SPEC HACK]
+
+This is a chunk of code from 456.hmmer:
+
+int f(int M, int *mc, int *mpp, int *tpmm, int *ip, int *tpim, int *dpp,
+     int *tpdm, int xmb, int *bp, int *ms) {
+ int k, sc;
+ for (k = 1; k <= M; k++) {
+     mc[k] = mpp[k-1]   + tpmm[k-1];
+     if ((sc = ip[k-1]  + tpim[k-1]) > mc[k])  mc[k] = sc;
+     if ((sc = dpp[k-1] + tpdm[k-1]) > mc[k])  mc[k] = sc;
+     if ((sc = xmb  + bp[k])         > mc[k])  mc[k] = sc;
+     mc[k] += ms[k];
+   }
+}
+
+It is very profitable for this benchmark to turn the conditional stores to mc[k]
+into a conditional move (select instr in IR) and allow the final store to do the
+store.  See GCC PR27313 for more details.  Note that this is valid to xform even
+with the new C++ memory model, since mc[k] is previously loaded and later
+stored.
 
 //===---------------------------------------------------------------------===//