add a horrible note

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@64719 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Target/X86/README.txt b/lib/Target/X86/README.txt
index 9a380ac..e8c2561 100644 (file)
--- a/lib/Target/X86/README.txt
+++ b/lib/Target/X86/README.txt
@@ -1777,3 +1777,44 @@ LBB1_2:  ## overflow
 it would be nice to produce "into" someday.
 
 //===---------------------------------------------------------------------===//
+
+This code:
+
+void vec_mpys1(int y[], const int x[], int scaler) {
+int i;
+for (i = 0; i < 150; i++)
+ y[i] += (((long long)scaler * (long long)x[i]) >> 31);
+}
+
+Compiles to this loop with GCC 3.x:
+
+.L5:
+       movl    %ebx, %eax
+       imull   (%edi,%ecx,4)
+       shrdl   $31, %edx, %eax
+       addl    %eax, (%esi,%ecx,4)
+       incl    %ecx
+       cmpl    $149, %ecx
+       jle     .L5
+
+llvm-gcc compiles it to the much uglier:
+
+LBB1_1:        ## bb1
+       movl    24(%esp), %eax
+       movl    (%eax,%edi,4), %ebx
+       movl    %ebx, %ebp
+       imull   %esi, %ebp
+       movl    %ebx, %eax
+       mull    %ecx
+       addl    %ebp, %edx
+       sarl    $31, %ebx
+       imull   %ecx, %ebx
+       addl    %edx, %ebx
+       shldl   $1, %eax, %ebx
+       movl    20(%esp), %eax
+       addl    %ebx, (%eax,%edi,4)
+       incl    %edi
+       cmpl    $150, %edi
+       jne     LBB1_1  ## bb1
+
+//===---------------------------------------------------------------------===//