From: Chris Lattner <sabre@nondot.org>
Date: Mon, 15 Jan 2007 06:25:39 +0000 (+0000)
Subject: add some notes
X-Git-Url: http://plrg.eecs.uci.edu/git/?a=commitdiff_plain;h=7ace299bdca4c5397eb7962dc5360e693ba77e5d;p=oota-llvm.git

add some notes


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@33228 91177308-0d34-0410-b5e6-96231b3b80d8
---

diff --git a/lib/Target/X86/README.txt b/lib/Target/X86/README.txt
index 7e365203a80..bea5b8dbca4 100644
--- a/lib/Target/X86/README.txt
+++ b/lib/Target/X86/README.txt
@@ -762,3 +762,71 @@ int f(char *p) {
 We should inline lrintf and probably other libc functions.
 
 //===---------------------------------------------------------------------===//
+
+Start using the flags more.  For example, compile:
+
+int add_zf(int *x, int y, int a, int b) {
+     if ((*x += y) == 0)
+          return a;
+     else
+          return b;
+}
+
+to:
+       addl    %esi, (%rdi)
+       movl    %edx, %eax
+       cmovne  %ecx, %eax
+       ret
+instead of:
+
+_add_zf:
+        addl (%rdi), %esi
+        movl %esi, (%rdi)
+        testl %esi, %esi
+        cmove %edx, %ecx
+        movl %ecx, %eax
+        ret
+
+and:
+
+int add_zf(int *x, int y, int a, int b) {
+     if ((*x + y) < 0)
+          return a;
+     else
+          return b;
+}
+
+to:
+
+add_zf:
+        addl    (%rdi), %esi
+        movl    %edx, %eax
+        cmovns  %ecx, %eax
+        ret
+
+instead of:
+
+_add_zf:
+        addl (%rdi), %esi
+        testl %esi, %esi
+        cmovs %edx, %ecx
+        movl %ecx, %eax
+        ret
+
+//===---------------------------------------------------------------------===//
+
+This:
+#include <math.h>
+int foo(double X) { return isnan(X); }
+
+compiles to (-m64):
+
+_foo:
+        pxor %xmm1, %xmm1
+        ucomisd %xmm1, %xmm0
+        setp %al
+        movzbl %al, %eax
+        ret
+
+the pxor is not needed, we could compare the value against itself.
+