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

[Chris Lattner]

Changes in directory llvm/lib/Target/PowerPC:

README.txt updated: 1.69 -> 1.70
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Log message:

add a note


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Diffs of the changes:  (+41 -0)

 README.txt |   41 +++++++++++++++++++++++++++++++++++++++++
 1 files changed, 41 insertions(+)


Index: llvm/lib/Target/PowerPC/README.txt
diff -u llvm/lib/Target/PowerPC/README.txt:1.69 llvm/lib/Target/PowerPC/README.txt:1.70
--- llvm/lib/Target/PowerPC/README.txt:1.69	Mon Mar  6 22:42:59 2006
+++ llvm/lib/Target/PowerPC/README.txt	Tue Mar  7 18:25:47 2006
@@ -466,3 +466,44 @@
 
 This only matters when we start generating cr logical ops.
 
+===-------------------------------------------------------------------------===
+
+We should compile these two functions to the same thing:
+
+#include <stdlib.h>
+void f(int a, int b, int *P) {
+  *P = (a-b)>=0?(a-b):(b-a);
+}
+void g(int a, int b, int *P) {
+  *P = abs(a-b);
+}
+
+Further, they should compile to something better than:
+
+_g:
+        subf r2, r4, r3
+        subfic r3, r2, 0
+        cmpwi cr0, r2, -1
+        bgt cr0, LBB2_2 ; entry
+LBB2_1: ; entry
+        mr r2, r3
+LBB2_2: ; entry
+        stw r2, 0(r5)
+        blr
+
+GCC produces:
+
+_g:
+        subf r4,r4,r3
+        srawi r2,r4,31
+        xor r0,r2,r4
+        subf r0,r2,r0
+        stw r0,0(r5)
+        blr
+
+... which is much nicer.
+
+This theoretically may help improve twolf slightly (used in dimbox.c:142?).
+
+===-------------------------------------------------------------------------===
+