Author: lattner
Date: Fri Dec 28 16:30:05 2007
New Revision: 45388
URL: http://llvm.org/viewvc/llvm-project?rev=45388&view=rev
Log:
add a note.
Modified:
llvm/trunk/lib/Target/README.txt
Modified: llvm/trunk/lib/Target/README.txt
URL:
http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/README.txt?rev=45388&r1=45387&r2=45388&view=diff
==============================================================================
--- llvm/trunk/lib/Target/README.txt (original)
+++ llvm/trunk/lib/Target/README.txt Fri Dec 28 16:30:05 2007
@@ -480,6 +480,38 @@
}
}
+//===---------------------------------------------------------------------===//
+
+We should investigate an instruction sinking pass. Consider this silly
+example in pic mode:
+
+#include <assert.h>
+void foo(int x) {
+ assert(x);
+ //...
+}
+
+we compile this to:
+_foo:
+ subl $28, %esp
+ call "L1$pb"
+"L1$pb":
+ popl %eax
+ cmpl $0, 32(%esp)
+ je LBB1_2 # cond_true
+LBB1_1: # return
+ # ...
+ addl $28, %esp
+ ret
+LBB1_2: # cond_true
+...
+
+The PIC base computation (call+popl) is only used on one path through the
+code, but is currently always computed in the entry block. It would be
+better to sink the picbase computation down into the block for the
+assertion, as it is the only one that uses it. This happens for a lot of
+code with early outs.
+In this case, whole-function-isel would also handle this.
//===---------------------------------------------------------------------===//
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