[PATCH] D45218: [if-converter] Handle BB that terminate in ret during diamond conversion

[Valentin Churavy via Phabricator via llvm-commits]

vchuravy created this revision.
vchuravy added reviewers: kparzysz, iteratee.
Herald added a subscriber: nemanjai.

While I was updating the Julia frontend for LLVM 6.0.0 I encountered a situation
where we emitted code that triggered the diamond conversion in if-converter while
TrueBB and FalseBB terminated in `ret`.

    BB
  /    \

TrueBB  FalseBB

  ...     ...
  ret     ret

The if-converter pass decided that this was a valid structure to do a diamond conversion on
and this triggered an assertion later on since if-converter implictly assumed that TrueBB and
FalseBB terminate in a branch or a fall-trough. Since the situation described above is equivalent
to:

    BB
  /    \

TrueBB  FalseBB

  ...     ...
    \     /
       BB
       ret

I think it reasonable for if-converter to do a diamond conversion on and so this PR adds a bit handling for
return (treating returns as a form of an unconditional branch).

This fixes https://bugs.llvm.org/show_bug.cgi?id=36825


Repository:
  rL LLVM

https://reviews.llvm.org/D45218

Files:
  lib/CodeGen/IfConversion.cpp
  test/CodeGen/MIR/PowerPC/ifcvt-diamond-ret.mir


Index: test/CodeGen/MIR/PowerPC/ifcvt-diamond-ret.mir
===================================================================
--- /dev/null
+++ test/CodeGen/MIR/PowerPC/ifcvt-diamond-ret.mir
@@ -0,0 +1,36 @@
+# RUN: llc -mtriple=powerpc64le-unknown-linux-gnu -run-pass=if-converter %s -o - | FileCheck %s
+---
+name:           foo
+body:           |
+  bb.0:
+  liveins: $x0, $x3
+  successors: %bb.1(0x40000000), %bb.2(0x40000000)
+
+  dead renamable $x3 = ANDIo8 killed renamable $x3, 1, implicit-def dead $cr0, implicit-def $cr0gt
+  $cr2lt = CROR $cr0gt, $cr0gt
+  BCn killed renamable $cr2lt, %bb.2
+  B %bb.1
+
+  bb.1:
+    renamable $x3 = LIS8 4096
+    MTLR8 $x0, implicit-def $lr8
+    BLR8 implicit $lr8, implicit $rm, implicit $x3
+
+  bb.2:
+    renamable $x3 = LIS8 4096
+    MTLR8 $x0, implicit-def $lr8
+    BLR8 implicit $lr8, implicit $rm, implicit $x3
+...
+
+# Diamond testcase with branches terminating in returns,
+
+# CHECK: body:             |          
+# CHECK:  bb.0:
+# CHECK:    dead renamable $x3 = ANDIo8 killed renamable $x3, 1, implicit-def dead $cr0, implicit-def $cr0gt
+# CHECK:    $cr2lt = CROR $cr0gt, $cr0gt
+# CHECK:    renamable $x3 = LIS8 4096
+# CHECK:    MTLR8 $x0, implicit-def $lr8
+# CHECK:    BCLRn $cr2lt, implicit $lr8, implicit $rm, implicit $x3
+# CHECK:    BCLR $cr2lt, implicit $lr8, implicit $rm, implicit $x3
+
+
Index: lib/CodeGen/IfConversion.cpp
===================================================================
--- lib/CodeGen/IfConversion.cpp
+++ lib/CodeGen/IfConversion.cpp
@@ -646,7 +646,8 @@
     if (TII->DefinesPredicate(*TIB, PredDefs))
       return false;
     // If we get all the way to the branch instructions, don't count them.
-    if (!TIB->isBranch())
+    // We treat returns as unconditional branches
+    if (!(TIB->isBranch() || TIB->isReturn()))
       ++Dups1;
     ++TIB;
     ++FIB;
@@ -685,7 +686,7 @@
       break;
     // We have to verify that any branch instructions are the same, and then we
     // don't count them toward the # of duplicate instructions.
-    if (!RTIE->isBranch())
+    if (!(RTIE->isBranch() || RTIE->isReturn()))
       ++Dups2;
     ++RTIE;
     ++RFIE;


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