[SystemZ] Improve CCMask optimization (#171137)

This commit addresses a shortcoming in the implementation of
`combineBR_CCMASK` and `combineSELECT_CCMASK`. In cases where
`combineCCMask` was able to reduce the ccmask going into the select or
branch to either true (`ccvalid`) or false (`0`), a trivial instruction
would be emitted (i.e. either a select that would only ever select one
side, or a conditional branch with `true` or `false` as the branch
condition).
This led under certain circumstances to, e.g., `BRC` instructions being
emitted that triggered an assert in the AsmPrinter meant to exclude such
branch conditions.
For the select case, this commit introduces an early bailout that simply
returns the value that would "always" be selected. For the branch case,
the commit introduces an additional guard that prevents the DAGCombine
from taking effect, thereby preventing the illegal instruction from
being emitted.

Author: dominik-steenken

llvm/lib/Target/SystemZ/SystemZISelLowering.cpp

@@ -8887,7 +8887,12 @@ SDValue SystemZTargetLowering::combineBR_CCMASK(SDNode *N,
   int CCMaskVal = CCMask->getZExtValue();
   SDValue Chain = N->getOperand(0);
   SDValue CCReg = N->getOperand(4);
-  if (combineCCMask(CCReg, CCValidVal, CCMaskVal, DAG))
+  // If combineCMask was able to merge or simplify ccvalid or ccmask, re-emit
+  // the modified BR_CCMASK with the new values.
+  // In order to avoid conditional branches with full or empty cc masks, do not
+  // do this if ccmask is 0 or equal to ccvalid.
+  if (combineCCMask(CCReg, CCValidVal, CCMaskVal, DAG) && CCMaskVal != 0 &&
+      CCMaskVal != CCValidVal)
     return DAG.getNode(SystemZISD::BR_CCMASK, SDLoc(N), N->getValueType(0),
                        Chain,
                        DAG.getTargetConstant(CCValidVal, SDLoc(N), MVT::i32),
@@ -8974,6 +8979,13 @@ SDValue SystemZTargetLowering::combineSELECT_CCMASK(
       IsCombinedCCReg = true;
     }
   }
+  // If the condition is trivially false or trivially true after
+  // combineCCMask, just collapse this SELECT_CCMASK to the indicated value
+  // (possibly modified by constructCCSDValsFromSELECT).
+  if (CCMaskVal == 0)
+    return FalseVal;
+  if (CCMaskVal == CCValidVal)
+    return TrueVal;
 
   if (IsCombinedCCReg)
     return DAG.getNode(

llvm/test/CodeGen/SystemZ/condfolding.ll

@@ -0,0 +1,28 @@
+; Test that a conditional branch with a discoverably trivial condition
+; does not result in an invalid conditional branch instruction.
+;
+; RUN: llc < %s -verify-machineinstrs -mtriple=s390x-linux-gnu -mcpu=z13 \
+; RUN:   --stop-after=systemz-isel | FileCheck %s
+
+@g_1 = dso_local local_unnamed_addr global i64 0, align 8
+@g_2 = dso_local local_unnamed_addr global i32 0, align 4
+
+define dso_local void @f1() local_unnamed_addr #1 {
+entry:
+;CHECK-LABEL: f1
+;CHECK-NOT: BRC 14, 0, %bb.2
+  %0 = load i64, ptr @g_1, align 8
+  %tobool.not = icmp eq i64 %0, 0
+  %sub.i = select i1 %tobool.not, i8 4, i8 3
+  %conv1 = zext nneg i8 %sub.i to i32
+  store i32 %conv1, ptr @g_2, align 4
+  %.pr = load i32, ptr @g_2, align 4
+  %tobool5.not = icmp eq i32 %.pr, 0
+  br i1 %tobool5.not, label %for.cond, label %lbl_1
+
+lbl_1:
+  br label %lbl_1
+
+for.cond:
+  br label %for.cond
+}