release/18.x: [X86] Fix miscompile in combineShiftRightArithmetic

[AtariDreams]

When folding (ashr (shl, x, c1), c2) we need to treat c1 and c2 as unsigned to find out if the combined shift should be a left or right shift.
Also do an early out during pre-legalization in case c1 and c2 has different types, as that otherwise complicated the comparison of c1 and c2 a bit.

(cherry picked from commit 3e6e54e)

[llvmbot]

@llvm/pr-subscribers-backend-x86

Author: AtariDreams (AtariDreams)

Changes

When folding (ashr (shl, x, c1), c2) we need to treat c1 and c2 as unsigned to find out if the combined shift should be a left or right shift.
Also do an early out during pre-legalization in case c1 and c2 has differet types, as that otherwise complicated the comparison of c1 and c2 a bit.

(cherry picked from commit 3e6e54e)

---

Full diff: https://github.com/llvm/llvm-project/pull/86728.diff

2 Files Affected:

• (modified) llvm/lib/Target/X86/X86ISelLowering.cpp (+16-13)
• (modified) llvm/test/CodeGen/X86/sar_fold.ll (+41)

diff --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
index 9e64726fb6fff7..71fc6b5047eaa9 100644
--- a/llvm/lib/Target/X86/X86ISelLowering.cpp
+++ b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -47035,10 +47035,13 @@ static SDValue combineShiftRightArithmetic(SDNode *N, SelectionDAG &DAG,
   if (SDValue V = combineShiftToPMULH(N, DAG, Subtarget))
     return V;
 
-  // fold (ashr (shl, a, [56,48,32,24,16]), SarConst)
-  // into (shl, (sext (a), [56,48,32,24,16] - SarConst)) or
-  // into (lshr, (sext (a), SarConst - [56,48,32,24,16]))
-  // depending on sign of (SarConst - [56,48,32,24,16])
+  // fold (SRA (SHL X, ShlConst), SraConst)
+  // into (SHL (sext_in_reg X), ShlConst - SraConst)
+  //   or (sext_in_reg X)
+  //   or (SRA (sext_in_reg X), SraConst - ShlConst)
+  // depending on relation between SraConst and ShlConst.
+  // We only do this if (Size - ShlConst) is equal to 8, 16 or 32. That allows
+  // us to do the sext_in_reg from corresponding bit.
 
   // sexts in X86 are MOVs. The MOVs have the same code size
   // as above SHIFTs (only SHIFT on 1 has lower code size).
@@ -47054,29 +47057,29 @@ static SDValue combineShiftRightArithmetic(SDNode *N, SelectionDAG &DAG,
   SDValue N00 = N0.getOperand(0);
   SDValue N01 = N0.getOperand(1);
   APInt ShlConst = N01->getAsAPIntVal();
-  APInt SarConst = N1->getAsAPIntVal();
+  APInt SraConst = N1->getAsAPIntVal();
   EVT CVT = N1.getValueType();
 
-  if (SarConst.isNegative())
+  if (CVT != N01.getValueType())
+    return SDValue();
+  if (SraConst.isNegative())
     return SDValue();
 
   for (MVT SVT : { MVT::i8, MVT::i16, MVT::i32 }) {
     unsigned ShiftSize = SVT.getSizeInBits();
-    // skipping types without corresponding sext/zext and
-    // ShlConst that is not one of [56,48,32,24,16]
+    // Only deal with (Size - ShlConst) being equal to 8, 16 or 32.
     if (ShiftSize >= Size || ShlConst != Size - ShiftSize)
       continue;
     SDLoc DL(N);
     SDValue NN =
         DAG.getNode(ISD::SIGN_EXTEND_INREG, DL, VT, N00, DAG.getValueType(SVT));
-    SarConst = SarConst - (Size - ShiftSize);
-    if (SarConst == 0)
+    if (SraConst.eq(ShlConst))
       return NN;
-    if (SarConst.isNegative())
+    if (SraConst.ult(ShlConst))
       return DAG.getNode(ISD::SHL, DL, VT, NN,
-                         DAG.getConstant(-SarConst, DL, CVT));
+                         DAG.getConstant(ShlConst - SraConst, DL, CVT));
     return DAG.getNode(ISD::SRA, DL, VT, NN,
-                       DAG.getConstant(SarConst, DL, CVT));
+                       DAG.getConstant(SraConst - ShlConst, DL, CVT));
   }
   return SDValue();
 }
diff --git a/llvm/test/CodeGen/X86/sar_fold.ll b/llvm/test/CodeGen/X86/sar_fold.ll
index 21655e19440afe..0f1396954b03a1 100644
--- a/llvm/test/CodeGen/X86/sar_fold.ll
+++ b/llvm/test/CodeGen/X86/sar_fold.ll
@@ -44,3 +44,44 @@ define i32 @shl24sar25(i32 %a) #0 {
   %2 = ashr exact i32 %1, 25
   ret i32 %2
 }
+
+define void @shl144sar48(ptr %p) #0 {
+; CHECK-LABEL: shl144sar48:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    movl {{[0-9]+}}(%esp), %eax
+; CHECK-NEXT:    movswl (%eax), %ecx
+; CHECK-NEXT:    movl %ecx, %edx
+; CHECK-NEXT:    sarl $31, %edx
+; CHECK-NEXT:    shldl $2, %ecx, %edx
+; CHECK-NEXT:    shll $2, %ecx
+; CHECK-NEXT:    movl %ecx, 12(%eax)
+; CHECK-NEXT:    movl %edx, 16(%eax)
+; CHECK-NEXT:    movl $0, 8(%eax)
+; CHECK-NEXT:    movl $0, 4(%eax)
+; CHECK-NEXT:    movl $0, (%eax)
+; CHECK-NEXT:    retl
+  %a = load i160, ptr %p
+  %1 = shl i160 %a, 144
+  %2 = ashr exact i160 %1, 46
+  store i160 %2, ptr %p
+  ret void
+}
+
+define void @shl144sar2(ptr %p) #0 {
+; CHECK-LABEL: shl144sar2:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    movl {{[0-9]+}}(%esp), %eax
+; CHECK-NEXT:    movswl (%eax), %ecx
+; CHECK-NEXT:    shll $14, %ecx
+; CHECK-NEXT:    movl %ecx, 16(%eax)
+; CHECK-NEXT:    movl $0, 8(%eax)
+; CHECK-NEXT:    movl $0, 12(%eax)
+; CHECK-NEXT:    movl $0, 4(%eax)
+; CHECK-NEXT:    movl $0, (%eax)
+; CHECK-NEXT:    retl
+  %a = load i160, ptr %p
+  %1 = shl i160 %a, 144
+  %2 = ashr exact i160 %1, 2
+  store i160 %2, ptr %p
+  ret void
+}

[tstellar]

@phoebewang What do you think about backporting this?

[phoebewang]

@phoebewang What do you think about backporting this?

I didn't review on it. Maybe @topperc can evaluate it.

[topperc]

@phoebewang What do you think about backporting this?

I didn't review on it. Maybe @topperc can evaluate it.

I think this is ok to backport.

[tstellar]

Hi @AtariDreams (or anyone else). If you would like to add a note about this fix in the release notes (completely optional). Please reply to this comment with a one or two sentence description of the fix. When you are done, please add the release:note label to this PR.