[llvm-commits] CVS: llvm/lib/Target/PowerPC/README.txt

[Chris Lattner]

Changes in directory llvm/lib/Target/PowerPC:

README.txt updated: 1.49 -> 1.50
---
Log message:

example nate pointed out


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Diffs of the changes:  (+33 -0)

 README.txt |   33 +++++++++++++++++++++++++++++++++
 1 files changed, 33 insertions(+)


Index: llvm/lib/Target/PowerPC/README.txt
diff -u llvm/lib/Target/PowerPC/README.txt:1.49 llvm/lib/Target/PowerPC/README.txt:1.50
--- llvm/lib/Target/PowerPC/README.txt:1.49	Mon Jan 30 20:55:28 2006
+++ llvm/lib/Target/PowerPC/README.txt	Tue Jan 31 01:16:34 2006
@@ -427,3 +427,36 @@
 
 without the lwz/stw's.
 
+===-------------------------------------------------------------------------===
+
+Compile this:
+
+int foo(int a) {
+  int b = (a < 8);
+  if (b) {
+    return b * 3;     // ignore the fact that this is always 3.
+  } else {
+    return 2;
+  }
+}
+
+into something not this:
+
+_foo:
+1)      cmpwi cr7, r3, 8
+        mfcr r2, 1
+        rlwinm r2, r2, 29, 31, 31
+1)      cmpwi cr0, r3, 7
+        bgt cr0, LBB1_2 ; UnifiedReturnBlock
+LBB1_1: ; then
+        rlwinm r2, r2, 0, 31, 31
+        mulli r3, r2, 3
+        blr
+LBB1_2: ; UnifiedReturnBlock
+        li r3, 2
+        blr
+
+In particular, the two compares (marked 1) could be shared by reversing one.
+This could be done in the dag combiner, by swapping a BR_CC when a SETCC of the
+same operands (but backwards) exists.  In this case, this wouldn't save us 
+anything though, because the compares still wouldn't be shared.