mirror of
https://github.com/c64scene-ar/llvm-6502.git
synced 2024-12-24 06:30:19 +00:00
A few missed optimizations; the last one could have a significant impact on
code with lots of bitfields. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@95809 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
parent
e555c9f4a5
commit
11d91e80b4
@ -1868,3 +1868,69 @@ carried over to machine instructions. Asm printer (or JIT) can use this
|
||||
information to add the "lock" prefix.
|
||||
|
||||
//===---------------------------------------------------------------------===//
|
||||
|
||||
_Bool bar(int *x) { return *x & 1; }
|
||||
|
||||
define zeroext i1 @bar(i32* nocapture %x) nounwind readonly {
|
||||
entry:
|
||||
%tmp1 = load i32* %x ; <i32> [#uses=1]
|
||||
%and = and i32 %tmp1, 1 ; <i32> [#uses=1]
|
||||
%tobool = icmp ne i32 %and, 0 ; <i1> [#uses=1]
|
||||
ret i1 %tobool
|
||||
}
|
||||
|
||||
bar: # @bar
|
||||
# BB#0: # %entry
|
||||
movl 4(%esp), %eax
|
||||
movb (%eax), %al
|
||||
andb $1, %al
|
||||
movzbl %al, %eax
|
||||
ret
|
||||
|
||||
Missed optimization: should be movl+andl.
|
||||
|
||||
//===---------------------------------------------------------------------===//
|
||||
|
||||
Consider the following two functions compiled with clang:
|
||||
_Bool foo(int *x) { return !(*x & 4); }
|
||||
unsigned bar(int *x) { return !(*x & 4); }
|
||||
|
||||
foo:
|
||||
movl 4(%esp), %eax
|
||||
testb $4, (%eax)
|
||||
sete %al
|
||||
movzbl %al, %eax
|
||||
ret
|
||||
|
||||
bar:
|
||||
movl 4(%esp), %eax
|
||||
movl (%eax), %eax
|
||||
shrl $2, %eax
|
||||
andl $1, %eax
|
||||
xorl $1, %eax
|
||||
ret
|
||||
|
||||
The second function generates more code even though the two functions are
|
||||
are functionally identical.
|
||||
|
||||
//===---------------------------------------------------------------------===//
|
||||
|
||||
Take the following C code:
|
||||
int x(int y) { return (y & 63) << 14; }
|
||||
|
||||
Code produced by gcc:
|
||||
andl $63, %edi
|
||||
sall $14, %edi
|
||||
movl %edi, %eax
|
||||
ret
|
||||
|
||||
Code produced by clang:
|
||||
shll $14, %edi
|
||||
movl %edi, %eax
|
||||
andl $1032192, %eax
|
||||
ret
|
||||
|
||||
The code produced by gcc is 3 bytes shorter. This sort of construct often
|
||||
shows up with bitfields.
|
||||
|
||||
//===---------------------------------------------------------------------===//
|
||||
|
Loading…
Reference in New Issue
Block a user