[LLVMbugs] [Bug 21790] New: wrong load instruction type generated for vector FP add (x86)

bugzilla-daemon at llvm.org bugzilla-daemon at llvm.org
Tue Dec 9 08:49:05 PST 2014


            Bug ID: 21790
           Summary: wrong load instruction type generated for vector FP
                    add (x86)
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: spatel+llvm at rotateright.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

In the following test case, we want to load the 2 lower floats in a v4f32 and
add them to some other v4f32:

$ cat wrong_load.ll 
define <4 x float> @add_2_floats(float* nocapture %p, <4 x float> %x) {
  %f1 = load float* %p
  %vecins1 = insertelement <4 x float> undef, float %f1, i32 0
  %add.ptr = getelementptr float* %p, i32 1
  %f2 = load float* %add.ptr
  %vecins2 = insertelement <4 x float> %vecins1, float %f2, i32 1
  %y = fadd <4 x float> %vecins2, %x
  ret <4 x float> %y

$ ./llc wrong_load.ll -o -
    movq    (%rdi), %xmm1   <--- integer load
    addps    %xmm1, %xmm0    <--- floating-point math op


We're generating an i64 load instead of the FP instruction for loading 2
floats: 'movlps'. This may cause a domain-crossing penalty.

The bug is located in X86ISelLowering's "EltsFromConsecutiveLoads()" function.

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