2013-08-29 07:04:41 +08:00
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; RUN: opt < %s -instcombine -S | FileCheck %s
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; CHECK: select
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; CHECK: select
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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.
llvm-svn: 68142
2009-04-01 04:42:45 +08:00
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; Make sure instcombine don't fold select into operands. We don't want to emit
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; select of two integers unless it's selecting 0 / 1.
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define i32 @t1(i32 %c, i32 %x) nounwind {
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%t1 = icmp eq i32 %c, 0
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%t2 = lshr i32 %x, 18
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%t3 = select i1 %t1, i32 %t2, i32 %x
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ret i32 %t3
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}
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define i32 @t2(i32 %c, i32 %x) nounwind {
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%t1 = icmp eq i32 %c, 0
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%t2 = and i32 %x, 18
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%t3 = select i1 %t1, i32 %t2, i32 %x
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ret i32 %t3
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}
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