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

[Chris Lattner]

Changes in directory llvm/lib/Target/X86:

README.txt updated: 1.127 -> 1.128
---
Log message:

Testcase noticed from PR906: http://llvm.org/PR906 


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Diffs of the changes:  (+34 -0)

 README.txt |   34 ++++++++++++++++++++++++++++++++++
 1 files changed, 34 insertions(+)


Index: llvm/lib/Target/X86/README.txt
diff -u llvm/lib/Target/X86/README.txt:1.127 llvm/lib/Target/X86/README.txt:1.128
--- llvm/lib/Target/X86/README.txt:1.127	Mon Sep 11 17:57:51 2006
+++ llvm/lib/Target/X86/README.txt	Mon Sep 11 18:00:56 2006
@@ -594,3 +594,37 @@
 cmp eax, 6
 jz label
 
+//===---------------------------------------------------------------------===//
+
+Compile:
+int %test(ulong *%tmp) {
+        %tmp = load ulong* %tmp         ; <ulong> [#uses=1]
+        %tmp.mask = shr ulong %tmp, ubyte 50            ; <ulong> [#uses=1]
+        %tmp.mask = cast ulong %tmp.mask to ubyte               ; <ubyte> [#uses=1]
+        %tmp2 = and ubyte %tmp.mask, 3          ; <ubyte> [#uses=1]
+        %tmp2 = cast ubyte %tmp2 to int         ; <int> [#uses=1]
+        ret int %tmp2
+}
+
+to:
+
+_test:
+        movl 4(%esp), %eax
+        movl 4(%eax), %eax
+        shrl $18, %eax
+        andl $3, %eax
+        ret
+
+instead of:
+
+_test:
+        movl 4(%esp), %eax
+        movl 4(%eax), %eax
+        shrl $18, %eax
+        # TRUNCATE movb %al, %al
+        andb $3, %al
+        movzbl %al, %eax
+        ret
+
+This saves a movzbl, and saves a truncate if it doesn't get coallesced right.
+This is a simple DAGCombine to propagate the zext through the and.