Commit Graph

1 Commits

Author SHA1 Message Date
Benjamin Kramer
9c64030445 Emit a more efficient magic number multiplication for exact sdivs.
We have to do this in DAGBuilder instead of DAGCombiner, because the exact bit is lost after building.

  struct foo { char x[24]; };
  long bar(struct foo *a, struct foo *b) { return a-b; }
is now compiled into
  movl	4(%esp), %eax
  subl	8(%esp), %eax
  sarl	$3, %eax
  imull	$-1431655765, %eax, %eax
instead of
  movl	4(%esp), %eax
  subl	8(%esp), %eax
  movl	$715827883, %ecx
  imull	%ecx
  movl	%edx, %eax
  shrl	$31, %eax
  sarl	$2, %edx
  addl	%eax, %edx
  movl	%edx, %eax

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@134695 91177308-0d34-0410-b5e6-96231b3b80d8
2011-07-08 10:31:30 +00:00