[LLVMbugs] [Bug 23627] New: Regression: ARM jump table miscompilation

[bugzilla-daemon at llvm.org]

https://llvm.org/bugs/show_bug.cgi?id=23627

            Bug ID: 23627
           Summary: Regression: ARM jump table miscompilation
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: Backend: ARM
          Assignee: unassignedbugs at nondot.org
          Reporter: peter at pcc.me.uk
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

Created attachment 14366
  --> https://llvm.org/bugs/attachment.cgi?id=14366&action=edit
source

Given the attached hist.cc, after reverting r237972:

$ clang -march=armv7-a -mtune=generic-armv7-a -mfpu=vfpv3-d16
-mfloat-abi=softfp -mthumb -target arm-linux-androideabi -Oz  
-fomit-frame-pointer  -fno-exceptions -fno-rtti -fno-threadsafe-statics
-std=gnu++11 -S -w hist.cc

produces the attached hist.s. The miscompiled code starts on line 564:

@ BB#26:
        adr.w   r0, .LJTI1_0
        add.w   r0, r0, r1, lsl #2
.LCPI1_48:
        tbh     [r0, r1, lsl #1]
        .align  2
@ BB#27:
.LCPI1_49:
        .long   600000                  @ 0x927c0
@ BB#28:
.LJTI1_0:
        .short  (.LBB1_29-(.LCPI1_48+4))/2
        .short  (.LBB1_32-(.LCPI1_48+4))/2
        .short  (.LBB1_42-(.LCPI1_48+4))/2
        .short  (.LBB1_38-(.LCPI1_48+4))/2
        .short  (.LBB1_41-(.LCPI1_48+4))/2

In this code the tbh instruction ends up loading from .LJTI_0 + (r1 << 2) + (r1
<< 1) instead of the intended .LJTI0 + (r1 << 1).

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