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Move and clarify note.

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@85334 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Bill Wendling 2009-10-27 22:48:31 +00:00
parent 3dab277857
commit 5a56927345
2 changed files with 33 additions and 31 deletions
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33
lib/Target/README.txt
View File

@ -1600,3 +1600,36 @@ in test/Transforms/SCCP/ipsccp-basic.ll:test5b.
//===---------------------------------------------------------------------===//
int func(int a, int b) { if (a & 0x80) b |= 0x80; else b &= ~0x80; return b; }
Generates this:
define i32 @func(i32 %a, i32 %b) nounwind readnone ssp {
entry:
%0 = and i32 %a, 128 ; <i32> [#uses=1]
%1 = icmp eq i32 %0, 0 ; <i1> [#uses=1]
%2 = or i32 %b, 128 ; <i32> [#uses=1]
%3 = and i32 %b, -129 ; <i32> [#uses=1]
%b_addr.0 = select i1 %1, i32 %3, i32 %2 ; <i32> [#uses=1]
ret i32 %b_addr.0
}
However, it's functionally equivalent to:
b = (b & ~0x80) | (a & 0x80);
Which generates this:
define i32 @func(i32 %a, i32 %b) nounwind readnone ssp {
entry:
%0 = and i32 %b, -129 ; <i32> [#uses=1]
%1 = and i32 %a, 128 ; <i32> [#uses=1]
%2 = or i32 %0, %1 ; <i32> [#uses=1]
ret i32 %2
}
This can be generalized for other forms:
b = (b & ~0x80) | (a & 0x40) << 1;
//===---------------------------------------------------------------------===//

31
lib/Target/X86/README.txt
View File

@ -1954,34 +1954,3 @@ carried over to machine instructions. Asm printer (or JIT) can use this
information to add the "lock" prefix.
//===---------------------------------------------------------------------===//
int func(int a, int b) { if (a & 0x80) b |= 0x80; else b &= ~0x80; return b; }
Current:
movb %sil, %al
andb $127, %sil
orb $-128, %al
testb %dil, %dil
js LBB1_2
movb %sil, %al
LBB1_2:
movsbl %al, %eax
Better:
movl %esi, %eax
orl $-128, %eax
andl $127, %esi
testb %dil, %dil
cmovns %esi, %eax
movsbl %al,%eax
Best (recognize this as 'b = (b & ~0x80) | (a & 0x80)'):
andb $-128, %dil
andb $127, %sil
orb %dil, %sil
movsbl %sil, %eax