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

[Nate Begeman]

Changes in directory llvm/lib/Target/PowerPC:

README.txt updated: 1.46 -> 1.47
---
Log message:

Add a couple more things to the readme.


---
Diffs of the changes:  (+53 -0)

 README.txt |   53 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 files changed, 53 insertions(+)


Index: llvm/lib/Target/PowerPC/README.txt
diff -u llvm/lib/Target/PowerPC/README.txt:1.46 llvm/lib/Target/PowerPC/README.txt:1.47
--- llvm/lib/Target/PowerPC/README.txt:1.46	Wed Jan 18 20:09:38 2006
+++ llvm/lib/Target/PowerPC/README.txt	Fri Jan 27 19:22:10 2006
@@ -194,6 +194,59 @@
 
 ===-------------------------------------------------------------------------===
 
+176.gcc contains a bunch of code like this (this occurs dozens of times):
+
+int %test(uint %mode.0.i.0) {
+        %tmp.79 = cast uint %mode.0.i.0 to sbyte        ; <sbyte> [#uses=1]
+        %tmp.80 = cast sbyte %tmp.79 to int             ; <int> [#uses=1]
+        %tmp.81 = shl int %tmp.80, ubyte 16             ; <int> [#uses=1]
+        %tmp.82 = and int %tmp.81, 16711680
+        ret int %tmp.82
+}
+
+which we compile to:
+
+_test:
+        extsb r2, r3
+        rlwinm r3, r2, 16, 8, 15
+        blr
+
+The extsb is obviously dead.  This can be handled by a future thing like 
+MaskedValueIsZero that checks to see if bits are ever demanded (in this case, 
+the sign bits are never used, so we can fold the sext_inreg to nothing).
+
+I'm seeing code like this:
+
+        srwi r3, r3, 16
+        extsb r3, r3
+        rlwimi r4, r3, 16, 8, 15
+
+in which the extsb is preventing the srwi from being nuked.
+
+===-------------------------------------------------------------------------===
+
+Another example that occurs is:
+
+uint %test(int %specbits.6.1) {
+        %tmp.2540 = shr int %specbits.6.1, ubyte 11     ; <int> [#uses=1]
+        %tmp.2541 = cast int %tmp.2540 to uint          ; <uint> [#uses=1]
+        %tmp.2542 = shl uint %tmp.2541, ubyte 13        ; <uint> [#uses=1]
+        %tmp.2543 = and uint %tmp.2542, 8192            ; <uint> [#uses=1]
+        ret uint %tmp.2543
+}
+
+which we codegen as:
+
+l1_test:
+        srawi r2, r3, 11
+        rlwinm r3, r2, 13, 18, 18
+        blr
+
+the srawi can be nuked by turning the SAR into a logical SHR (the sext bits are 
+dead), which I think can then be folded into the rlwinm.
+
+===-------------------------------------------------------------------------===
+
 Compile offsets from allocas:
 
 int *%test() {