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671f7eb81b
This patch helps avoid false dependencies on undef registers by updating the machine instructions' undef operand to use a register that the instruction is truly dependent on, or use a register with clearance higher than Pref. Pseudo example: loop: xmm0 = ... xmm1 = vcvtsi2sdl eax, xmm0<undef> ... = inst xmm0 jmp loop In this example, selecting xmm0 as the undef register creates false dependency between loop iterations. This false dependency cannot be solved by inserting an xor before vcvtsi2sdl because xmm0 is alive at the point of the vcvtsi2sdl instruction. Selecting a different register instead of xmm0, especially a register that is not used in the loop, will eliminate this problem. Differential Revision: https://reviews.llvm.org/D22466 llvm-svn: 278321
46 lines
1.8 KiB
LLVM
46 lines
1.8 KiB
LLVM
; RUN: llc %s -mattr=+avx -o - | FileCheck %s
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; PR21743.
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target triple = "x86_64-pc-win32-elf"
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; Check that copy propagation conservatively assumes that undef register
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; can be rewritten by the backend to break false dependencies for the
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; hardware.
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; In this function we are in this situation:
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; reg1 = copy reg2
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; = inst reg2<undef>
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; reg2 = copy reg1
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; Copy propagation used to remove the last copy.
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; This is incorrect because the undef flag on reg2 in inst, allows next
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; passes to put whatever trashed value in reg2 that may help.
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; In practice we end up with this code:
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; reg1 = copy reg2
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; reg2 = 0
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; = inst reg2<undef>
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; reg2 = copy reg1
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; Therefore, removing the last copy is wrong.
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;
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; CHECK-LABEL: foo:
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; CHECK: movl $339752784, %e[[INDIRECT_CALL1:[a-z]+]]
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; CHECK: callq *%r[[INDIRECT_CALL1]]
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; Copy the result in a temporary.
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; Note: Technically the regalloc could have been smarter and this move not required,
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; which would have hidden the bug.
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; CHECK: vmovapd %xmm0, [[TMP:%xmm[0-9]+]]
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; Crush xmm0.
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; CHECK-NEXT: vxorps %xmm0, %xmm0, %xmm0
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; CHECK: movl $339772768, %e[[INDIRECT_CALL2:[a-z]+]]
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; Set TMP in the first argument of the second call.
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; CHECK-NEXT: vmovapd [[TMP]], %xmm0
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; CHECK: callq *%r[[INDIRECT_CALL2]]
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; CHECK: retq
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define double @foo(i64 %arg) {
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top:
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%tmp = call double inttoptr (i64 339752784 to double (double, double)*)(double 1.000000e+00, double 0.000000e+00)
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tail call void asm sideeffect "", "x,~{xmm1},~{xmm2},~{xmm3},~{xmm4},~{xmm5},~{xmm6},~{xmm7},~{xmm8},~{xmm9},~{xmm10},~{xmm11},~{xmm12},~{xmm13},~{xmm14},~{xmm15},~{dirflag},~{fpsr},~{flags}"(double %tmp)
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%tmp1 = sitofp i64 %arg to double
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call void inttoptr (i64 339772768 to void (double, double)*)(double %tmp, double %tmp1)
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%tmp3 = fadd double %tmp1, %tmp
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ret double %tmp3
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}
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