PNaCl: Disable parts of InstCombine that introduce *.with.overflow intrinsics

PNaCl: Disable parts of InstCombine that introduce *.with.overflow intrinsics

Change the PNaCl ABI checker to disallow these intrinsics.

Note that I had originally intended to commit this before my earlier
change (https://codereview.chromium.org/15688011) that enables the
ExpandArithWithOverflow pass.  Enabling ExpandArithWithOverflow
without changing InstCombine causes ExpandArithWithOverflow to fail on
some of the *.with.overflow intrinsic calls that InstCombine
introduces.  This change therefore fixes some breakage.

BUG=https://code.google.com/p/nativeclient/issues/detail?id=3434
TEST=PNaCl toolchain trybots + GCC torture tests + LLVM test suite

Committed: https://gerrit.chromium.org/gerrit/gitweb?p=native_client/pnacl-llvm.git;a=commit;h=8d01804

[eliben, 2013-05-28 22:59:42]

Can you explain here or (preferably) in the issue why the extra breakage? It
would be really great if we could end up doing this without changing things
inside InstCombine. Such localmods are non-trivial to maintain. Perhaps this
customization can be upstreamed somehow?

[Mark Seaborn, 2013-05-28 23:02:59]

On 28 May 2013 15:59, <eliben@chromium.org> wrote:

> Can you explain here or (preferably) in the issue why the extra breakage?


The reference to breakage in the commit message is because I accidentally
committed these changes in the wrong order.

It would be really great if we could end up doing this without changing
> things
> inside InstCombine. Such localmods are non-trivial to maintain. Perhaps
> this
> customization can be upstreamed somehow?
>

It's explained in the issue:
https://code.google.com/p/nativeclient/issues/detail?id=3434

"Uses of the with.overflow intrinsics are introduced in the following
places:
 a) In the LLVM optimiser, by the InstCombine pass.  (See
lib/Transforms/InstCombine/InstCombineCompares.cpp.)  This recognises the C
idiom for addition overflow checks.  We could disable this part of
InstCombine."

Changing InstCombine is the easiest way to prevent it from introducing
these uses of *.with.overflow intrinsics.  It's tricky to expand out
*.with.overflow in the general case.

Mark

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[eliben, 2013-05-28 23:13:20]

lgtm

[Mark Seaborn, 2013-05-29 01:53:18]

Committed patchset #1 manually as r8d01804 (presubmit successful).