[llvm-commits] CVS: llvm/lib/Target/X86/README.txt

[Chris Lattner]

Changes in directory llvm/lib/Target/X86:

README.txt updated: 1.167 -> 1.168
---
Log message:

add some notes


---
Diffs of the changes:  (+28 -0)

 README.txt |   28 ++++++++++++++++++++++++++++
 1 files changed, 28 insertions(+)


Index: llvm/lib/Target/X86/README.txt
diff -u llvm/lib/Target/X86/README.txt:1.167 llvm/lib/Target/X86/README.txt:1.168
--- llvm/lib/Target/X86/README.txt:1.167	Sat May  5 17:10:24 2007
+++ llvm/lib/Target/X86/README.txt	Wed May  9 19:08:04 2007
@@ -1094,5 +1094,33 @@
 has this xform, but it is currently disabled until the alignment fields of 
 the load/store nodes are trustworthy.
 
+//===---------------------------------------------------------------------===//
 
+Sometimes it is better to codegen subtractions from a constant (e.g. 7-x) with
+a neg instead of a sub instruction.  Consider:
+
+int test(char X) { return 7-X; }
+
+we currently produce:
+_test:
+        movl $7, %eax
+        movsbl 4(%esp), %ecx
+        subl %ecx, %eax
+        ret
+
+We would use one fewer register if codegen'd as:
+
+        movsbl 4(%esp), %eax
+	neg %eax
+        add $7, %eax
+        ret
+
+Note that this isn't beneficial if the load can be folded into the sub.  In
+this case, we want a sub:
+
+int test(int X) { return 7-X; }
+_test:
+        movl $7, %eax
+        subl 4(%esp), %eax
+        ret