[llvm] r190309 - [ARMv8] Prevent generation of deprecated IT blocks on ARMv8 in Thumb mode.

[Artyom Skrobov]

Hello Hal,

The meaningful part of the change to IfConversion is as follows:

--- a/lib/CodeGen/IfConversion.cpp
+++ b/lib/CodeGen/IfConversion.cpp
@@ -682,16 +682,15 @@ void IfConverter::ScanInstructions(BBInfo &BBI) {
         BBI.IsUnpredicable = true;
         return;
       }
+    } else {
+      // A conditional branch is not predicable, but it may be eliminated.
+      continue;
     }
 
+
     if (BBI.ClobbersPred && !isPredicated) {
       // Predicate modification instruction should end the block (except
for
       // already predicated instructions and end of block branches).
-      if (isCondBr) {
-        // A conditional branch is not predicable, but it may be
eliminated.
-        continue;
-      }
-
       // Predicate may have been modified, the subsequent (currently)
       // unpredicated instructions cannot be correctly predicated.
       BBI.IsUnpredicable = true;

That is, skipping over the conditional branch instruction (terminating the
BB) whether or not this instruction is predicated.

This is because in ARMv8 Thumb, the conditional branches are non-predicable
-- despite being conditional.

Could this cause problems with the PPC backend?


________________________________________
From: Hal Finkel [hfinkel at anl.gov]
Sent: 21 November 2013 15:23
To: Joey Gouly
Cc: llvm-commits at cs.uiuc.edu
Subject: Re: [llvm] r190309 - [ARMv8] Prevent generation of deprecated IT
blocks  on ARMv8 in Thumb mode.

Joey,

This commit seems to have broken self hosting on PPC (only with an NDEBUG
build, which is why we did not catch it earlier). I can only suppose it is
the changes to lib/CodeGen/IfConversion.cpp that either caused the problem,
or triggered an existing PPC backend bug. Can you please explain the
rationale behind the IfConversion changes?

Thanks in advance,
Hal

----- Original Message -----
> From: "Joey Gouly" <joey.gouly at arm.com>
> To: "Joey Gouly" <Joey.Gouly at arm.com>, llvm-commits at cs.uiuc.edu
> Sent: Monday, September 9, 2013 9:48:02 AM
> Subject: RE: [llvm] r190309 - [ARMv8] Prevent generation of deprecated IT
blocks  on ARMv8 in Thumb mode.
>
> > Author: joey
> > Date: Mon Sep  9 09:21:49 2013
> > New Revision: 190309
> >
> > URL: http://llvm.org/viewvc/llvm-project?rev=190309&view=rev
> > Log:
> > [ARMv8] Prevent generation of deprecated IT blocks on ARMv8 in
> > Thumb mode.
> > IT blocks can only be one instruction lonf, and can only contain a
> > subset
> of
> > the 16 instructions.
>
> Sorry, the commit message here should have read "the 16-bit
> instructions".
>
>
>
>
>
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>

--
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory