[llvm] f6515b0 - [PowerPC] Do not fold `cmp(d|w)` and `subf` instruction to `subf.` if `nsw` is not present

[Kai Luo via llvm-commits]

Author: Kai Luo
Date: 2021-01-04T07:54:15Z
New Revision: f6515b05205d4324d174dd1f7455c6c8e6671e6b

URL: https://github.com/llvm/llvm-project/commit/f6515b05205d4324d174dd1f7455c6c8e6671e6b
DIFF: https://github.com/llvm/llvm-project/commit/f6515b05205d4324d174dd1f7455c6c8e6671e6b.diff

LOG: [PowerPC] Do not fold `cmp(d|w)` and `subf` instruction to `subf.` if `nsw` is not present

In `PPCInstrInfo::optimizeCompareInstr` we seek opportunities to fold `cmp(d|w)` and `subf` as an `subf.`. However, if `subf.` gets overflow, `cr0` can't reflect the correct order, violating the semantics of `cmp(d|w)`.

Fixed https://bugs.llvm.org/show_bug.cgi?id=47830.

Reviewed By: #powerpc, nemanjai

Differential Revision: https://reviews.llvm.org/D90156

Added: 
    

Modified: 
    llvm/lib/Target/PowerPC/PPCInstrInfo.cpp
    llvm/test/CodeGen/PowerPC/pr47830.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Target/PowerPC/PPCInstrInfo.cpp b/llvm/lib/Target/PowerPC/PPCInstrInfo.cpp
index deac690a7611..75a498b807cd 100644
--- a/llvm/lib/Target/PowerPC/PPCInstrInfo.cpp
+++ b/llvm/lib/Target/PowerPC/PPCInstrInfo.cpp
@@ -2141,6 +2141,14 @@ bool PPCInstrInfo::optimizeCompareInstr(MachineInstr &CmpInstr, Register SrcReg,
   if (NewOpC == -1)
     return false;
 
+  // This transformation should not be performed if `nsw` is missing and is not
+  // `equalityOnly` comparison. Since if there is overflow, sub_lt, sub_gt in
+  // CRReg do not reflect correct order. If `equalityOnly` is true, sub_eq in
+  // CRReg can reflect if compared values are equal, this optz is still valid.
+  if (!equalityOnly && (NewOpC == PPC::SUBF_rec || NewOpC == PPC::SUBF8_rec) &&
+      Sub && !Sub->getFlag(MachineInstr::NoSWrap))
+    return false;
+
   // If we have SUB(r1, r2) and CMP(r2, r1), the condition code based on CMP
   // needs to be updated to be based on SUB.  Push the condition code
   // operands to OperandsToUpdate.  If it is safe to remove CmpInstr, the

diff  --git a/llvm/test/CodeGen/PowerPC/pr47830.ll b/llvm/test/CodeGen/PowerPC/pr47830.ll
index be61a81462c2..bd320907a341 100644
--- a/llvm/test/CodeGen/PowerPC/pr47830.ll
+++ b/llvm/test/CodeGen/PowerPC/pr47830.ll
@@ -5,8 +5,9 @@
 define i64 @f(i64 %a, i64 %b) {
 ; CHECK-LABEL: f:
 ; CHECK:       # %bb.0:
-; CHECK-NEXT:    sub. r3, r3, r4
-; CHECK-NEXT:    isellt r3, 0, r3
+; CHECK-NEXT:    sub r5, r3, r4
+; CHECK-NEXT:    cmpd r3, r4
+; CHECK-NEXT:    isellt r3, 0, r5
 ; CHECK-NEXT:    blr
   %c = icmp slt i64 %a, %b
   %d = sub i64 %a, %b