[MIPS] Modify LongBranch expansion to work with sandboxing

test/MC/Mips/nacl-long-branch.ll

+; RUN: llc -filetype=obj -sfi-load -sfi-store -sfi-stack -sfi-branch -sfi-data \
+; RUN: -force-mips-long-branch -O3 -mtriple mipsel-none-nacl %s -o - \

[Mark Seaborn, 2013/10/24 23:38:43]

Can you indent these continuation lines for readability?
i.e. "RUN:     -force-mips..."

[petarj, 2013/11/21 18:23:42]

Done.

+; RUN: | llvm-objdump -triple mipsel -disassemble -no-show-raw-insn - \
+; RUN: | FileCheck %s
+
+
+declare void @f()
+
+define void @long_branch(i32 %arg) {
+entry:
+  %cmp = icmp eq i32 %arg, 0
+  br i1 %cmp, label %if.then, label %if.end
+if.then:
+  tail call void @f()
+  br label %if.end
+if.end:
+  ret void
+}
+
+; This file tests that MipsLongBranchPass works with sandboxing.
+; The nonsandboxed code of the previous function is:
+
+; long_branch:
+;       0:       lui     $2, 0
+;       4:       addiu   $2, $2, 0
+;       8:       addiu   $sp, $sp, -24
+;       c:       sw      $ra, 20($sp)
+;      10:       bne     $4, $zero, 20

[Mark Seaborn, 2013/10/24 23:38:43]

Might it be useful to add CHECKs for this case too (via a second RUN above), to
check your assumptions?

[petarj, 2013/11/21 18:23:42]

Done.

+;      14:       addu    $gp, $2, $25
+;      18:       lw      $25, 0($gp)
+;      1c:       jalr    $25
+;      20:       nop
+;      24:       lw      $ra, 20($sp)
+;      28:       addiu   $sp, $sp, 24
+;      2c:       jr      $ra
+;      30:       nop
+
+; We pass -force-mips-long-branch option to llc to make sure that bne
+; instruction is expanded into long branch. We specifically test that
+; in the expanded sandboxed long branch sequence the offset that LUi/ADDiu

[Mark Seaborn, 2013/10/24 23:38:43]

Can you add a comment explaining the arithmetic for this check?

Presumably it is:
  0x80 - 0x40 = 64
where 0x40 and 0x80 are addresses and 64 is in the "addiu" at 0x40.

It took me a while to understand what is going on here.

[petarj, 2013/11/21 18:23:42]

Done.

+; instructions create is the correct offset from the ADDiu instruction to the
+; branch target (which is lw $ra, 20($sp) in this case).
+
+;                0:       lui     $2, 0
+;                4:       addiu   $2, $2, 0
+;                8:       addiu   $sp, $sp, -24
+;                c:       and     $sp, $sp, $15
+;               10:       sw      $ra, 20($sp)
+;               14:       beq     $4, $zero, 80
+;               18:       addu    $gp, $2, $25
+;               1c:       nop
+;               20:       addiu   $sp, $sp, -8
+;               24:       and     $sp, $sp, $15
+;               28:       sw      $ra, 0($sp)
+;               2c:       nop
+;               30:       nop
+;               34:       nop
+; CHECK:        38:       bal     8
+; CHECK-NEXT:   3c:       lui     $1, 0
+; CHECK-NEXT:   40:       addiu   $1, $1, 64
+;               44:       addu    $1, $ra, $1
+;               48:       lw      $ra, 0($sp)
+;               4c:       nop
+;               50:       addiu   $sp, $sp, 8
+;               54:       and     $sp, $sp, $15
+;               58:       and     $1, $1, $14
+;               5c:       jr      $1

[Mark Seaborn, 2013/10/24 23:38:43]

The instructions from "bal 8" down to here are part of the converted 'long
branch' sequence, aren't they?  Shouldn't they have CHECKs too (but without
checking their hex offsets)?

[petarj, 2013/11/21 18:23:42]

Done.

+;               60:       nop
+;               64:       and     $gp, $gp, $15
+;               68:       lw      $25, 0($gp)
+;               6c:       nop
+;               70:       nop
+;               74:       and     $25, $25, $14
+;               78:       jalr    $25
+;               7c:       nop
+; CHECK:        80:       lw      $ra, 20($sp)
+;               84:       addiu   $sp, $sp, 24
+;               88:       and     $sp, $sp, $15
+;               8c:       nop
+;               90:       and     $ra, $ra, $14
+;               94:       jr      $ra
+;               98:       nop
+