[DAG] Peek through trunc when combining select into shifts.

This fixes a regression in D127115 Depends on D127115 Reviewed By: nikic Differential Revision: https://reviews.llvm.org/D151916
llvm · Jun 23, 2023 · 34d8c5b · 34d8c5b
1 parent 65b710e
commit 34d8c5b
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Showing 2 changed files with 33 additions and 36 deletions.
diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -2485,6 +2485,17 @@ SDValue DAGCombiner::foldBinOpIntoSelect(SDNode *BO) {
   if (Sel.getOpcode() != ISD::SELECT || !Sel.hasOneUse()) {
     SelOpNo = 1;
     Sel = BO->getOperand(1);
+
+    // Peek through trunc to shift amount type.
+    if ((BinOpcode == ISD::SHL || BinOpcode == ISD::SRA ||
+         BinOpcode == ISD::SRL) && Sel.hasOneUse()) {
+      // This is valid when the truncated bits of x are already zero.
+      SDValue Op;
+      KnownBits Known;
+      if (isTruncateOf(DAG, Sel, Op, Known) &&
+          Known.countMaxActiveBits() < Sel.getScalarValueSizeInBits())
+        Sel = Op;
+    }
   }
 
   if (Sel.getOpcode() != ISD::SELECT || !Sel.hasOneUse())

diff --git a/llvm/test/CodeGen/X86/dagcombine-select.ll b/llvm/test/CodeGen/X86/dagcombine-select.ll
@@ -194,12 +194,10 @@ define i32 @sel_constants_shl_constant(i1 %cond) {
 define i32 @shl_constant_sel_constants(i1 %cond) {
 ; CHECK-LABEL: shl_constant_sel_constants:
 ; CHECK:       # %bb.0:
-; CHECK-NEXT:    movl %edi, %ecx
-; CHECK-NEXT:    andb $1, %cl
-; CHECK-NEXT:    xorb $3, %cl
-; CHECK-NEXT:    movl $1, %eax
-; CHECK-NEXT:    # kill: def $cl killed $cl killed $ecx
-; CHECK-NEXT:    shll %cl, %eax
+; CHECK-NEXT:    notb %dil
+; CHECK-NEXT:    movzbl %dil, %eax
+; CHECK-NEXT:    andl $1, %eax
+; CHECK-NEXT:    leal 4(,%rax,4), %eax
 ; CHECK-NEXT:    retq
   %sel = select i1 %cond, i32 2, i32 3
   %bo = shl i32 1, %sel
@@ -209,12 +207,10 @@ define i32 @shl_constant_sel_constants(i1 %cond) {
 define i32 @shl_constant_sel_setcc(i32 %a) {
 ; CHECK-LABEL: shl_constant_sel_setcc:
 ; CHECK:       # %bb.0:
-; CHECK-NEXT:    movl %edi, %ecx
-; CHECK-NEXT:    andb $1, %cl
-; CHECK-NEXT:    xorb $3, %cl
-; CHECK-NEXT:    movl $1, %eax
-; CHECK-NEXT:    # kill: def $cl killed $cl killed $ecx
-; CHECK-NEXT:    shll %cl, %eax
+; CHECK-NEXT:    xorl %eax, %eax
+; CHECK-NEXT:    testb $1, %dil
+; CHECK-NEXT:    sete %al
+; CHECK-NEXT:    leal 4(,%rax,4), %eax
 ; CHECK-NEXT:    retq
   %m = and i32 %a, 1
   %cond = icmp ne i32 %m, 0
@@ -226,12 +222,9 @@ define i32 @shl_constant_sel_setcc(i32 %a) {
 define i32 @lshr_constant_sel_constants(i1 %cond) {
 ; CHECK-LABEL: lshr_constant_sel_constants:
 ; CHECK:       # %bb.0:
-; CHECK-NEXT:    movl %edi, %ecx
-; CHECK-NEXT:    andb $1, %cl
-; CHECK-NEXT:    xorb $3, %cl
-; CHECK-NEXT:    movl $64, %eax
-; CHECK-NEXT:    # kill: def $cl killed $cl killed $ecx
-; CHECK-NEXT:    shrl %cl, %eax
+; CHECK-NEXT:    # kill: def $edi killed $edi def $rdi
+; CHECK-NEXT:    andl $1, %edi
+; CHECK-NEXT:    leal 8(,%rdi,8), %eax
 ; CHECK-NEXT:    retq
   %sel = select i1 %cond, i32 2, i32 3
   %bo = lshr i32 64, %sel
@@ -241,12 +234,9 @@ define i32 @lshr_constant_sel_constants(i1 %cond) {
 define i32 @lshr_constant_sel_setcc(i32 %a) {
 ; CHECK-LABEL: lshr_constant_sel_setcc:
 ; CHECK:       # %bb.0:
-; CHECK-NEXT:    movl %edi, %ecx
-; CHECK-NEXT:    andb $1, %cl
-; CHECK-NEXT:    xorb $3, %cl
-; CHECK-NEXT:    movl $64, %eax
-; CHECK-NEXT:    # kill: def $cl killed $cl killed $ecx
-; CHECK-NEXT:    shrl %cl, %eax
+; CHECK-NEXT:    # kill: def $edi killed $edi def $rdi
+; CHECK-NEXT:    andl $1, %edi
+; CHECK-NEXT:    leal 8(,%rdi,8), %eax
 ; CHECK-NEXT:    retq
   %m = and i32 %a, 1
   %cond = icmp ne i32 %m, 0
@@ -258,12 +248,10 @@ define i32 @lshr_constant_sel_setcc(i32 %a) {
 define i32 @ashr_constant_sel_constants(i1 %cond) {
 ; CHECK-LABEL: ashr_constant_sel_constants:
 ; CHECK:       # %bb.0:
-; CHECK-NEXT:    movl %edi, %ecx
-; CHECK-NEXT:    andb $1, %cl
-; CHECK-NEXT:    xorb $3, %cl
-; CHECK-NEXT:    movl $128, %eax
-; CHECK-NEXT:    # kill: def $cl killed $cl killed $ecx
-; CHECK-NEXT:    shrl %cl, %eax
+; CHECK-NEXT:    # kill: def $edi killed $edi def $rdi
+; CHECK-NEXT:    andl $1, %edi
+; CHECK-NEXT:    shll $4, %edi
+; CHECK-NEXT:    leal 16(%rdi), %eax
 ; CHECK-NEXT:    retq
   %sel = select i1 %cond, i32 2, i32 3
   %bo = ashr i32 128, %sel
@@ -273,12 +261,10 @@ define i32 @ashr_constant_sel_constants(i1 %cond) {
 define i32 @ashr_constant_sel_setcc(i32 %a) {
 ; CHECK-LABEL: ashr_constant_sel_setcc:
 ; CHECK:       # %bb.0:
-; CHECK-NEXT:    movl %edi, %ecx
-; CHECK-NEXT:    andb $1, %cl
-; CHECK-NEXT:    xorb $3, %cl
-; CHECK-NEXT:    movl $128, %eax
-; CHECK-NEXT:    # kill: def $cl killed $cl killed $ecx
-; CHECK-NEXT:    shrl %cl, %eax
+; CHECK-NEXT:    # kill: def $edi killed $edi def $rdi
+; CHECK-NEXT:    andl $1, %edi
+; CHECK-NEXT:    shll $4, %edi
+; CHECK-NEXT:    leal 16(%rdi), %eax
 ; CHECK-NEXT:    retq
   %m = and i32 %a, 1
   %cond = icmp ne i32 %m, 0