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
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