[llvm-commits] [llvm] r122495 - /llvm/trunk/lib/Target/X86/README.txt
Benjamin Kramer
benny.kra at googlemail.com
Thu Dec 23 07:07:02 PST 2010
Author: d0k
Date: Thu Dec 23 09:07:02 2010
New Revision: 122495
URL: http://llvm.org/viewvc/llvm-project?rev=122495&view=rev
Log:
Remove some obsolete README items, add a new one off the top of my head.
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=122495&r1=122494&r2=122495&view=diff
==============================================================================
--- llvm/trunk/lib/Target/X86/README.txt (original)
+++ llvm/trunk/lib/Target/X86/README.txt Thu Dec 23 09:07:02 2010
@@ -67,19 +67,6 @@
//===---------------------------------------------------------------------===//
-Compile this:
-_Bool f(_Bool a) { return a!=1; }
-
-into:
- movzbl %dil, %eax
- xorl $1, %eax
- ret
-
-(Although note that this isn't a legal way to express the code that llvm-gcc
-currently generates for that function.)
-
-//===---------------------------------------------------------------------===//
-
Some isel ideas:
1. Dynamic programming based approach when compile time if not an
@@ -394,72 +381,8 @@
//===---------------------------------------------------------------------===//
-Codegen:
-
-int f(int a, int b) {
- if (a == 4 || a == 6)
- b++;
- return b;
-}
-
-
-as:
-
-or eax, 2
-cmp eax, 6
-jz label
-
-//===---------------------------------------------------------------------===//
-
GCC's ix86_expand_int_movcc function (in i386.c) has a ton of interesting
-simplifications for integer "x cmp y ? a : b". For example, instead of:
-
-int G;
-void f(int X, int Y) {
- G = X < 0 ? 14 : 13;
-}
-
-compiling to:
-
-_f:
- movl $14, %eax
- movl $13, %ecx
- movl 4(%esp), %edx
- testl %edx, %edx
- cmovl %eax, %ecx
- movl %ecx, _G
- ret
-
-it could be:
-_f:
- movl 4(%esp), %eax
- sarl $31, %eax
- notl %eax
- addl $14, %eax
- movl %eax, _G
- ret
-
-etc.
-
-Another is:
-int usesbb(unsigned int a, unsigned int b) {
- return (a < b ? -1 : 0);
-}
-to:
-_usesbb:
- movl 8(%esp), %eax
- cmpl %eax, 4(%esp)
- sbbl %eax, %eax
- ret
-
-instead of:
-_usesbb:
- xorl %eax, %eax
- movl 8(%esp), %ecx
- cmpl %ecx, 4(%esp)
- movl $4294967295, %ecx
- cmovb %ecx, %eax
- ret
+simplifications for integer "x cmp y ? a : b".
//===---------------------------------------------------------------------===//
@@ -1868,3 +1791,28 @@
the load and the store which would prohibit narrowing.
//===---------------------------------------------------------------------===//
+
+This code:
+void foo(unsigned x) {
+ if (x == 0) bar();
+ else if (x == 1) qux();
+}
+
+currently compiles into:
+_foo:
+ movl 4(%esp), %eax
+ cmpl $1, %eax
+ je LBB0_3
+ testl %eax, %eax
+ jne LBB0_4
+
+the testl could be removed:
+_foo:
+ movl 4(%esp), %eax
+ cmpl $1, %eax
+ je LBB0_3
+ jb LBB0_4
+
+0 is the only unsigned number < 1.
+
+//===---------------------------------------------------------------------===//
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