[llvm-commits] [llvm] r60286 - /llvm/trunk/lib/Target/X86/README.txt

[Eli Friedman]

Author: efriedma
Date: Sun Nov 30 01:52:27 2008
New Revision: 60286

URL: http://llvm.org/viewvc/llvm-project?rev=60286&view=rev
Log:
A couple small cleanups, plus a new potential optimization.


Modified:
    llvm/trunk/lib/Target/X86/README.txt

Modified: llvm/trunk/lib/Target/X86/README.txt
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/X86/README.txt?rev=60286&r1=60285&r2=60286&view=diff

==============================================================================
--- llvm/trunk/lib/Target/X86/README.txt (original)
+++ llvm/trunk/lib/Target/X86/README.txt Sun Nov 30 01:52:27 2008
@@ -394,6 +394,10 @@
 However, if we care more about code size, then imull is better. It's two bytes
 shorter than movl + leal.
 
+On a Pentium M, both variants have the same characteristics with regard
+to throughput; however, the multiplication has a latency of four cycles, as
+opposed to two cycles for the movl+lea variant.
+
 //===---------------------------------------------------------------------===//
 
 __builtin_ffs codegen is messy.
@@ -628,9 +632,9 @@
 to grab the bytes from the next cacheline.
 
 532  532 0x3cfc movb     (1809(%esp, %esi), %bl   <<<--- spans 2 64 byte lines
-942  942 0x3d03 movl     %dh, (1809(%esp, %esi)                                                                          
-937  937 0x3d0a incl     %esi                           
-3    3   0x3d0b cmpb     %bl, %dl                                               
+942  942 0x3d03 movl     %dh, (1809(%esp, %esi)
+937  937 0x3d0a incl     %esi
+3    3   0x3d0b cmpb     %bl, %dl
 27   27  0x3d0d jnz      0x000062db <main+11707>
 
 //===---------------------------------------------------------------------===//
@@ -1790,3 +1794,25 @@
           (v8i8 (MMX_MOVDQ2Qrr VR128:$src))>;
 
 There are other cases in various td files.
+
+//===---------------------------------------------------------------------===//
+
+Take something like the following on x86-32:
+unsigned a(unsigned long long x, unsigned y) {return x % y;}
+
+We currently generate a libcall, but we really shouldn't: the expansion is
+shorter and likely faster than the libcall.  The expected code is something
+like the following:
+
+	movl	12(%ebp), %eax
+	movl	16(%ebp), %ecx
+	xorl	%edx, %edx
+	divl	%ecx
+	movl	8(%ebp), %eax
+	divl	%ecx
+	movl	%edx, %eax
+	ret
+
+A similar code sequence works for division.
+
+//===---------------------------------------------------------------------===//