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Remove some obsolete README items, add a new one off the top of my head.

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@122495 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Benjamin Kramer 2010-12-23 15:07:02 +00:00
parent 2902736a50
commit 3e4106105b
1 changed files with 26 additions and 78 deletions
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104
lib/Target/X86/README.txt
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@ -67,19 +67,6 @@ cmovs, we should expand to a conditional branch like GCC produces.
//===---------------------------------------------------------------------===//
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 @@ boundary to improve performance.
//===---------------------------------------------------------------------===//
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 @@ load, making it non-trivial to determine if there's anything between
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.
//===---------------------------------------------------------------------===//