llvm/test/Transforms/InstCombine/select-2.ll
Evan Cheng de62192843 Throttle back "fold select into operand" transformation. InstCombine should not generate selects of two constants unless they are selects of 0 and 1.
e.g.
define i32 @t1(i32 %c, i32 %x) nounwind {
       %t1 = icmp eq i32 %c, 0
       %t2 = lshr i32 %x, 18
       %t3 = select i1 %t1, i32 %t2, i32 %x
       ret i32 %t3
}

was turned into

define i32 @t2(i32 %c, i32 %x) nounwind {
       %t1 = icmp eq i32 %c, 0
       %t2 = select i1 %t1, i32 18, i32 0
       %t3 = lshr i32 %x, %t2
       ret i32 %t3
}

For most targets, that means materializing two constants and then a select. e.g. On x86-64

movl    %esi, %eax
shrl    $18, %eax
testl   %edi, %edi
cmovne  %esi, %eax
ret

=>

xorl    %eax, %eax
testl   %edi, %edi
movl    $18, %ecx
cmovne  %eax, %ecx
movl    %esi, %eax
shrl    %cl, %eax
ret

Also, the optimizer and codegen can reason about shl / and / add, etc. by a constant. This optimization will hinder optimizations using ComputeMaskedBits.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@68142 91177308-0d34-0410-b5e6-96231b3b80d8
2009-03-31 20:42:45 +00:00

19 lines
543 B
LLVM

; RUN: llvm-as < %s | opt -instcombine | llvm-dis | grep select | count 2
; Make sure instcombine don't fold select into operands. We don't want to emit
; select of two integers unless it's selecting 0 / 1.
define i32 @t1(i32 %c, i32 %x) nounwind {
%t1 = icmp eq i32 %c, 0
%t2 = lshr i32 %x, 18
%t3 = select i1 %t1, i32 %t2, i32 %x
ret i32 %t3
}
define i32 @t2(i32 %c, i32 %x) nounwind {
%t1 = icmp eq i32 %c, 0
%t2 = and i32 %x, 18
%t3 = select i1 %t1, i32 %t2, i32 %x
ret i32 %t3
}