add a note, this is why we're faster at SciMark-MonteCarlo with

SSE disabled.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@63751 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Target/X86/README-SSE.txt b/lib/Target/X86/README-SSE.txt
index bc51b53482430043fffb8202642b99d360cbf272..67cad42a3548a7281fb0978c8698bce5412460cd 100644 (file)
--- a/lib/Target/X86/README-SSE.txt
+++ b/lib/Target/X86/README-SSE.txt
@@ -912,3 +912,43 @@ since we know the stack slot is already zext'd.
 Consider using movlps instead of movsd to implement (scalar_to_vector (loadf64))
 when code size is critical. movlps is slower than movsd on core2 but it's one
 byte shorter.
+
+//===---------------------------------------------------------------------===//
+
+We should use a dynamic programming based approach to tell when using FPStack
+operations is cheaper than SSE.  SciMark montecarlo contains code like this
+for example:
+
+double MonteCarlo_num_flops(int Num_samples) {
+    return ((double) Num_samples)* 4.0;
+}
+
+In fpstack mode, this compiles into:
+
+LCPI1_0:                                       
+       .long   1082130432      ## float 4.000000e+00
+_MonteCarlo_num_flops:
+       subl    $4, %esp
+       movl    8(%esp), %eax
+       movl    %eax, (%esp)
+       fildl   (%esp)
+       fmuls   LCPI1_0
+       addl    $4, %esp
+       ret
+        
+in SSE mode, it compiles into significantly slower code:
+
+_MonteCarlo_num_flops:
+       subl    $12, %esp
+       cvtsi2sd        16(%esp), %xmm0
+       mulsd   LCPI1_0, %xmm0
+       movsd   %xmm0, (%esp)
+       fldl    (%esp)
+       addl    $12, %esp
+       ret
+
+There are also other cases in scimark where using fpstack is better, it is
+cheaper to do fld1 than load from a constant pool for example, so
+"load, add 1.0, store" is better done in the fp stack, etc.
+
+//===---------------------------------------------------------------------===//