llvm/sdiv-exact.ll at fa3dc9ffbb5d8b9bb742940388dd14fc5f282362 - llvm - Gitea: Git with a cup of tea

RPCS3/llvm

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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

19 lines

351 B

LLVM

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 ; 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
 }