[X86][MC] Check AdSize16 for 16-bit addressing (#99761)

Summary: Fixes: #99735

Test Plan: 

Reviewers: 

Subscribers: 

Tasks: 

Tags: 


Differential Revision: https://phabricator.intern.facebook.com/D60251195

Author: phoebewang

llvm/lib/Target/X86/MCTargetDesc/X86MCCodeEmitter.cpp

@@ -691,9 +691,12 @@ void X86MCCodeEmitter::emitMemModRMByte(
 
   unsigned BaseRegNo = BaseReg ? getX86RegNum(Base) : -1U;
 
+  bool IsAdSize16 = STI.hasFeature(X86::Is32Bit) &&
+                    (TSFlags & X86II::AdSizeMask) == X86II::AdSize16;
+
   // 16-bit addressing forms of the ModR/M byte have a different encoding for
   // the R/M field and are far more limited in which registers can be used.
-  if (X86_MC::is16BitMemOperand(MI, Op, STI)) {
+  if (IsAdSize16 || X86_MC::is16BitMemOperand(MI, Op, STI)) {
     if (BaseReg) {
       // For 32-bit addressing, the row and column values in Table 2-2 are
       // basically the same. It's AX/CX/DX/BX/SP/BP/SI/DI in that order, with

llvm/test/MC/X86/x86-32-coverage.s

@@ -10790,7 +10790,7 @@ btcl $4, (%eax)
           movdir64b 485498096, %ecx
 
 // CHECK: movdir64b 485498096, %cx
-// CHECK: # encoding: [0x67,0x66,0x0f,0x38,0xf8,0x0d,0xf0,0x1c,0xf0,0x1c]
+// CHECK: # encoding: [0x67,0x66,0x0f,0x38,0xf8,0x0e,0xf0,0x1c]
           movdir64b 485498096, %cx
 
 // CHECK: movdir64b (%edx), %eax
@@ -10877,6 +10877,10 @@ enqcmd  (%bx,%di), %di
 // CHECK: encoding: [0x67,0xf2,0x0f,0x38,0xf8,0x81,0xc0,0x1f]
 enqcmd  8128(%bx,%di), %ax
 
+// CHECK: enqcmd 485498096, %cx
+// CHECK: encoding: [0x67,0xf2,0x0f,0x38,0xf8,0x0e,0xf0,0x1c]
+enqcmd 485498096, %cx
+
 // CHECK: enqcmds (%bx,%di), %di
 // CHECK: encoding: [0x67,0xf3,0x0f,0x38,0xf8,0x39]
 enqcmds (%bx,%di), %di
@@ -10885,6 +10889,10 @@ enqcmds (%bx,%di), %di
 // CHECK: encoding: [0x67,0xf3,0x0f,0x38,0xf8,0x81,0xc0,0x1f]
 enqcmds 8128(%bx,%di), %ax
 
+// CHECK: enqcmds 485498096, %cx
+// CHECK: encoding: [0x67,0xf3,0x0f,0x38,0xf8,0x0e,0xf0,0x1c]
+enqcmds 485498096, %cx
+
 // CHECK: serialize
 // CHECK: encoding: [0x0f,0x01,0xe8]
 serialize