[DAGCombiner] Fix miscompile bug in combineShiftOfShiftedLogic (#89616)

Ensure that the sum of the shift amounts does not overflow the
shift amount type when combining shifts in combineShiftOfShiftedLogic.

Solves a miscompile bug found when testing the C23 BitInt feature.

Targets like X86 that only use an i8 for shift amounts after
legalization seems to be extra susceptible for bugs like this as it
isn't legal to shift more than 255 steps.

Author: bjope

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -9503,8 +9503,15 @@ static SDValue combineShiftOfShiftedLogic(SDNode *Shift, SelectionDAG &DAG) {
     if (ShiftAmtVal->getBitWidth() != C1Val.getBitWidth())
       return false;
 
+    // The fold is not valid if the sum of the shift values doesn't fit in the
+    // given shift amount type.
+    bool Overflow = false;
+    APInt NewShiftAmt = C1Val.uadd_ov(*ShiftAmtVal, Overflow);
+    if (Overflow)
+      return false;
+
     // The fold is not valid if the sum of the shift values exceeds bitwidth.
-    if ((*ShiftAmtVal + C1Val).uge(V.getScalarValueSizeInBits()))
+    if (NewShiftAmt.uge(V.getScalarValueSizeInBits()))
       return false;
 
     return true;

llvm/test/CodeGen/X86/shift-combine.ll

@@ -788,61 +788,31 @@ define <4 x i32> @or_tree_with_mismatching_shifts_vec_i32(<4 x i32> %a, <4 x i32
   ret <4 x i32> %r
 }
 
-; FIXME: Reproducer for a DAGCombiner::combineShiftOfShiftedLogic
-; bug. DAGCombiner need to check that the sum of the shift amounts fits in i8,
-; which is the legal type used to described X86 shift amounts. Verify that we
-; do not try to create a shift with 130+160 as shift amount, and verify that
-; the stored value do not depend on %a1.
+; Reproducer for a DAGCombiner::combineShiftOfShiftedLogic bug. DAGCombiner
+; need to check that the sum of the shift amounts fits in i8, which is the
+; legal type used to described X86 shift amounts. Verify that we do not try to
+; create a shift with 130+160 as shift amount, and verify that the stored
+; value do not depend on %a1.
 define void @combineShiftOfShiftedLogic(i128 %a1, i32 %a2, ptr %p) {
 ; X86-LABEL: combineShiftOfShiftedLogic:
 ; X86:       # %bb.0:
-; X86-NEXT:    pushl %ebx
-; X86-NEXT:    .cfi_def_cfa_offset 8
-; X86-NEXT:    pushl %edi
-; X86-NEXT:    .cfi_def_cfa_offset 12
-; X86-NEXT:    pushl %esi
-; X86-NEXT:    .cfi_def_cfa_offset 16
-; X86-NEXT:    .cfi_offset %esi, -16
-; X86-NEXT:    .cfi_offset %edi, -12
-; X86-NEXT:    .cfi_offset %ebx, -8
 ; X86-NEXT:    movl {{[0-9]+}}(%esp), %eax
 ; X86-NEXT:    movl {{[0-9]+}}(%esp), %ecx
-; X86-NEXT:    movl {{[0-9]+}}(%esp), %edx
-; X86-NEXT:    movl {{[0-9]+}}(%esp), %ebx
-; X86-NEXT:    movl {{[0-9]+}}(%esp), %edi
-; X86-NEXT:    movl %edi, %esi
-; X86-NEXT:    shldl $2, %ebx, %edi
-; X86-NEXT:    shldl $2, %edx, %ebx
-; X86-NEXT:    shrl $30, %esi
-; X86-NEXT:    orl {{[0-9]+}}(%esp), %esi
-; X86-NEXT:    shldl $2, %ecx, %edx
-; X86-NEXT:    shll $2, %ecx
-; X86-NEXT:    movl %edi, 16(%eax)
-; X86-NEXT:    movl %ebx, 12(%eax)
-; X86-NEXT:    movl %edx, 8(%eax)
-; X86-NEXT:    movl %ecx, 4(%eax)
-; X86-NEXT:    movl %esi, 20(%eax)
-; X86-NEXT:    movl $0, (%eax)
-; X86-NEXT:    popl %esi
-; X86-NEXT:    .cfi_def_cfa_offset 12
-; X86-NEXT:    popl %edi
-; X86-NEXT:    .cfi_def_cfa_offset 8
-; X86-NEXT:    popl %ebx
-; X86-NEXT:    .cfi_def_cfa_offset 4
+; X86-NEXT:    movl %eax, 20(%ecx)
+; X86-NEXT:    movl $0, 16(%ecx)
+; X86-NEXT:    movl $0, 12(%ecx)
+; X86-NEXT:    movl $0, 8(%ecx)
+; X86-NEXT:    movl $0, 4(%ecx)
+; X86-NEXT:    movl $0, (%ecx)
 ; X86-NEXT:    retl
 ;
 ; X64-LABEL: combineShiftOfShiftedLogic:
 ; X64:       # %bb.0:
 ; X64-NEXT:    # kill: def $edx killed $edx def $rdx
 ; X64-NEXT:    shlq $32, %rdx
-; X64-NEXT:    movq %rsi, %rax
-; X64-NEXT:    shrq $30, %rax
-; X64-NEXT:    orq %rdx, %rax
-; X64-NEXT:    shldq $34, %rdi, %rsi
-; X64-NEXT:    shlq $34, %rdi
-; X64-NEXT:    movq %rsi, 8(%rcx)
-; X64-NEXT:    movq %rdi, (%rcx)
-; X64-NEXT:    movq %rax, 16(%rcx)
+; X64-NEXT:    movq %rdx, 16(%rcx)
+; X64-NEXT:    movq $0, 8(%rcx)
+; X64-NEXT:    movq $0, (%rcx)
 ; X64-NEXT:    retq
   %zext1 = zext i128 %a1 to i192
   %zext2 = zext i32 %a2 to i192