[llvm-commits] [llvm] r77598 - /llvm/trunk/lib/Target/ARM/README.txt

[Chris Lattner]

Author: lattner
Date: Thu Jul 30 11:08:58 2009
New Revision: 77598

URL: http://llvm.org/viewvc/llvm-project?rev=77598&view=rev
Log:
add a random codegen deficiency.

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

Modified: llvm/trunk/lib/Target/ARM/README.txt
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/ARM/README.txt?rev=77598&r1=77597&r2=77598&view=diff

==============================================================================
--- llvm/trunk/lib/Target/ARM/README.txt (original)
+++ llvm/trunk/lib/Target/ARM/README.txt Thu Jul 30 11:08:58 2009
@@ -546,3 +546,49 @@
 
 Constant island pass should make use of full range SoImm values for LEApcrel.
 Be careful though as the last attempt caused infinite looping on lencod.
+
+//===---------------------------------------------------------------------===//
+
+Predication issue. This function:   
+
+extern unsigned array[ 128 ];
+int     foo( int x ) {
+  int     y;
+  y = array[ x & 127 ];
+  if ( x & 128 )
+     y = 123456789 & ( y >> 2 );
+  else
+     y = 123456789 & y;
+  return y;
+}
+
+compiles to:
+
+_foo:
+	and r1, r0, #127
+	ldr r2, LCPI1_0
+	ldr r2, [r2]
+	ldr r1, [r2, +r1, lsl #2]
+	mov r2, r1, lsr #2
+	tst r0, #128
+	moveq r2, r1
+	ldr r0, LCPI1_1
+	and r0, r2, r0
+	bx lr
+
+It would be better to do something like this, to fold the shift into the
+conditional move:
+
+	and r1, r0, #127
+	ldr r2, LCPI1_0
+	ldr r2, [r2]
+	ldr r1, [r2, +r1, lsl #2]
+	tst r0, #128
+	movne r1, r1, lsr #2
+	ldr r0, LCPI1_1
+	and r0, r1, r0
+	bx lr
+
+it saves an instruction and a register.
+
+//===---------------------------------------------------------------------===//