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

llvm-svn: 134695

1 files changed, 18 insertions, 0 deletions

diff --git a/llvm/test/CodeGen/X86/sdiv-exact.ll b/llvm/test/CodeGen/X86/sdiv-exact.ll
new file mode 100644
index 00000000000..48bb8836e89
--- /dev/null
+++ b/llvm/test/CodeGen/X86/sdiv-exact.ll
@@ -0,0 +1,18 @@
+; RUN: llc -march=x86 < %s | FileCheck %s
+
+define i32 @test1(i32 %x) {
+  %div = sdiv exact i32 %x, 25
+  ret i32 %div
+; CHECK: test1:
+; CHECK: imull	$-1030792151, 4(%esp)
+; CHECK-NEXT: ret
+}
+
+define i32 @test2(i32 %x) {
+  %div = sdiv exact i32 %x, 24
+  ret i32 %div
+; CHECK: test2:
+; CHECK: sarl	$3
+; CHECK-NEXT: imull	$-1431655765
+; CHECK-NEXT: ret
+}