Author: lattner
Date: Sun Jan 27 01:31:41 2008
New Revision: 46413
URL: http://llvm.org/viewvc/llvm-project?rev=46413&view=rev
Log:
add a note
Modified:
llvm/trunk/lib/Target/X86/README-SSE.txt
Modified: llvm/trunk/lib/Target/X86/README-SSE.txt
URL:
http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/X86/README-SSE.txt?rev=46413&r1=46412&r2=46413&view=diff
==============================================================================
--- llvm/trunk/lib/Target/X86/README-SSE.txt (original)
+++ llvm/trunk/lib/Target/X86/README-SSE.txt Sun Jan 27 01:31:41 2008
@@ -722,3 +722,42 @@
vec_align.ll without -relocation-model=static, you'll see what I mean.
//===---------------------------------------------------------------------===//
+
+We should lower store(fneg(load p), q) into an integer load+xor+store, which
+eliminates a constant pool load. For example, consider:
+
+define i64 @ccosf(float %z.0, float %z.1) nounwind readonly {
+entry:
+ %tmp6 = sub float -0.000000e+00, %z.1 ; <float> [#uses=1]
+ %tmp20 = tail call i64 @ccoshf( float %tmp6, float %z.0 ) nounwind
readonly ; <i64> [#uses=1]
+ ret i64 %tmp20
+}
+
+This currently compiles to:
+
+LCPI1_0: # <4 x float>
+ .long 2147483648 # float -0
+ .long 2147483648 # float -0
+ .long 2147483648 # float -0
+ .long 2147483648 # float -0
+_ccosf:
+ subl $12, %esp
+ movss 16(%esp), %xmm0
+ movss %xmm0, 4(%esp)
+ movss 20(%esp), %xmm0
+ xorps LCPI1_0, %xmm0
+ movss %xmm0, (%esp)
+ call L_ccoshf$stub
+ addl $12, %esp
+ ret
+
+Note the load into xmm0, then xor (to negate), then store. In PIC mode,
+this code computes the pic base and does two loads to do the constant pool
+load, so the improvement is much bigger.
+
+The tricky part about this xform is that the argument load/store isn't exposed
+until post-legalize, and at that point, the fneg has been custom expanded into
+an X86 fxor. This means that we need to handle this case in the x86 backend
+instead of in target independent code.
+
+//===---------------------------------------------------------------------===//
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