Author: lattner
Date: Thu Jan 10 12:25:41 2008
New Revision: 45837
URL: http://llvm.org/viewvc/llvm-project?rev=45837&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=45837&r1=45836&r2=45837&view=diff
==============================================================================
--- llvm/trunk/lib/Target/README.txt (original)
+++ llvm/trunk/lib/Target/README.txt Thu Jan 10 12:25:41 2008
@@ -545,3 +545,53 @@
instructions.
//===---------------------------------------------------------------------===//
+
+DAG Combiner should try to combine small loads into larger loads when
+profitable. For example, we compile this C++ example:
+
+struct THotKey { short Key; bool Control; bool Shift; bool Alt; };
+extern THotKey m_HotKey;
+THotKey GetHotKey () { return m_HotKey; }
+
+into (-O3 -fno-exceptions -static -fomit-frame-pointer):
+
+__Z9GetHotKeyv:
+ pushl %esi
+ movl 8(%esp), %eax
+ movb _m_HotKey+3, %cl
+ movb _m_HotKey+4, %dl
+ movb _m_HotKey+2, %ch
+ movw _m_HotKey, %si
+ movw %si, (%eax)
+ movb %ch, 2(%eax)
+ movb %cl, 3(%eax)
+ movb %dl, 4(%eax)
+ popl %esi
+ ret $4
+
+GCC produces:
+
+__Z9GetHotKeyv:
+ movl _m_HotKey, %edx
+ movl 4(%esp), %eax
+ movl %edx, (%eax)
+ movzwl _m_HotKey+4, %edx
+ movw %dx, 4(%eax)
+ ret $4
+
+The LLVM IR contains the needed alignment info, so we should be able to
+merge the loads and stores into 4-byte loads:
+
+ %struct.THotKey = type { i16, i8, i8, i8 }
+define void @_Z9GetHotKeyv(%struct.THotKey* sret %agg.result) nounwind {
+...
+ %tmp2 = load i16* getelementptr (@m_HotKey, i32 0, i32 0), align 8
+ %tmp5 = load i8* getelementptr (@m_HotKey, i32 0, i32 1), align 2
+ %tmp8 = load i8* getelementptr (@m_HotKey, i32 0, i32 2), align 1
+ %tmp11 = load i8* getelementptr (@m_HotKey, i32 0, i32 3), align 2
+
+Alternatively, we should use a small amount of base-offset alias analysis
+to make it so the scheduler doesn't need to hold all the loads in regs at
+once.
+
+//===---------------------------------------------------------------------===//
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