add a readme.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@114303 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Target/PowerPC/README.txt b/lib/Target/PowerPC/README.txt
index 3465779e344b4bee65b821254aea58f0af329f46..4c80225d041bfdfa8c3e999ccbc6fd0a5a9d9eca 100644 (file)
--- a/lib/Target/PowerPC/README.txt
+++ b/lib/Target/PowerPC/README.txt
@@ -37,6 +37,31 @@ _f3:
        ori r3, r2, 65535
        blr 
 
+===-------------------------------------------------------------------------===
+
+This code:
+
+unsigned add32carry(unsigned sum, unsigned x) {
+ unsigned z = sum + x;
+ if (sum + x < x)
+     z++;
+ return z;
+}
+
+Should compile to something like:
+
+       addc r3,r3,r4
+       addze r3,r3
+
+instead we get:
+
+       add r3, r4, r3
+       cmplw cr7, r3, r4
+       mfcr r4 ; 1
+       rlwinm r4, r4, 29, 31, 31
+       add r3, r3, r4
+
+Ick.
 
 ===-------------------------------------------------------------------------===