[llvm-commits] [llvm] r85334 - in /llvm/trunk/lib/Target: README.txt X86/README.txt

[Bill Wendling]

Author: void
Date: Tue Oct 27 17:48:31 2009
New Revision: 85334

URL: http://llvm.org/viewvc/llvm-project?rev=85334&view=rev
Log:
Move and clarify note.

Modified:
    llvm/trunk/lib/Target/README.txt
    llvm/trunk/lib/Target/X86/README.txt

Modified: llvm/trunk/lib/Target/README.txt
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/README.txt?rev=85334&r1=85333&r2=85334&view=diff

==============================================================================
--- llvm/trunk/lib/Target/README.txt (original)
+++ llvm/trunk/lib/Target/README.txt Tue Oct 27 17:48:31 2009
@@ -1600,3 +1600,36 @@
 
 //===---------------------------------------------------------------------===//
 
+int func(int a, int b) { if (a & 0x80) b |= 0x80; else b &= ~0x80; return b; }
+
+Generates this:
+
+define i32 @func(i32 %a, i32 %b) nounwind readnone ssp {
+entry:
+  %0 = and i32 %a, 128                            ; <i32> [#uses=1]
+  %1 = icmp eq i32 %0, 0                          ; <i1> [#uses=1]
+  %2 = or i32 %b, 128                             ; <i32> [#uses=1]
+  %3 = and i32 %b, -129                           ; <i32> [#uses=1]
+  %b_addr.0 = select i1 %1, i32 %3, i32 %2        ; <i32> [#uses=1]
+  ret i32 %b_addr.0
+}
+
+However, it's functionally equivalent to:
+
+         b = (b & ~0x80) | (a & 0x80);
+
+Which generates this:
+
+define i32 @func(i32 %a, i32 %b) nounwind readnone ssp {
+entry:
+  %0 = and i32 %b, -129                           ; <i32> [#uses=1]
+  %1 = and i32 %a, 128                            ; <i32> [#uses=1]
+  %2 = or i32 %0, %1                              ; <i32> [#uses=1]
+  ret i32 %2
+}
+
+This can be generalized for other forms:
+
+     b = (b & ~0x80) | (a & 0x40) << 1;
+
+//===---------------------------------------------------------------------===//

Modified: llvm/trunk/lib/Target/X86/README.txt
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/X86/README.txt?rev=85334&r1=85333&r2=85334&view=diff

==============================================================================
--- llvm/trunk/lib/Target/X86/README.txt (original)
+++ llvm/trunk/lib/Target/X86/README.txt Tue Oct 27 17:48:31 2009
@@ -1954,34 +1954,3 @@
 information to add the "lock" prefix.
 
 //===---------------------------------------------------------------------===//
-
-int func(int a, int b) { if (a & 0x80) b |= 0x80; else b &= ~0x80; return b; }
-
-Current:
-
-
-        movb    %sil, %al
-        andb    $127, %sil
-        orb     $-128, %al
-        testb   %dil, %dil
-        js      LBB1_2
-        movb    %sil, %al
-LBB1_2:
-        movsbl  %al, %eax
-
-
-Better:
-
-        movl    %esi, %eax
-        orl     $-128, %eax
-        andl    $127, %esi
-        testb   %dil, %dil
-        cmovns  %esi, %eax
-        movsbl  %al,%eax
-
-Best (recognize this as 'b = (b & ~0x80) | (a & 0x80)'):
-
-        andb    $-128, %dil
-        andb    $127, %sil
-        orb     %dil, %sil
-        movsbl  %sil, %eax