[X86] [MC] fixed non optimal encoding of instruction memory operand (PR24038)

[kbelochapka]

Summary

Fixed suboptimal encoding of instruction memory operand when assembler selected 32 bit fixup rather than 8 bit immediate for encoding memory offset value.

[craig.topper]

LGTM

lib/Target/X86/AsmParser/X86AsmParser.cpp

if (getLexer().isNot(AsmToken::LParen)) {
     SMLoc LParenLoc = Parser.getTok().getLoc();
     Parser.Lex(); // Eat the '('.
 
     if (getLexer().is(AsmToken::Percent) || getLexer().is(AsmToken::Comma)) {
       // Nothing to do here, fall into the code below with the '(' part of the
       // memory operand consumed.
     } else {
       SMLoc ExprEnd;
+      getLexer().UnLex(AsmToken(AsmToken::LParen, "("));
 
-      // It must be an parenthesized expression, parse it now.
-      if (getParser().parseParenExpression(Disp, ExprEnd))
+      // It must be either an parenthesized expression, or an expression that
+      // begins from a parenthesized expression, parse it now. Example: (1+2) or
+      // (1+2)+3
+      if (getParser().parseExpression(Disp, ExprEnd))
         return nullptr;
 
       // After parsing the base expression we could either have a parenthesized
       // memory address or not.  If not, return now.  If so, eat the (.
       if (getLexer().isNot(AsmToken::LParen)) {
         // Unless we have a segment register, treat this as an immediate.
         if (SegReg == 0)
           return X86Operand::CreateMem(getPointerWidth(), Disp, LParenLoc,

test/MC/X86/x86_64-encoding.s

 
 // CHECK: pinsrw $3, %ecx, %xmm5
 // CHECK: encoding: [0x66,0x0f,0xc4,0xe9,0x03]
           pinsrw $3, %ecx, %xmm5
 
 // CHECK: pinsrw $3, %ecx, %xmm5
 // CHECK: encoding: [0x66,0x0f,0xc4,0xe9,0x03]
           pinsrw $3, %rcx, %xmm5
+
+//CHECK   movq  12(%rdi), %rsi
+//CHECK   encoding: [0x48,0x8b,0x77,0x0c]
+    movq  16+0-4(%rdi),%rsi
+
+//CHECK   movq  12(%rdi), %rsi
+//CHECK   encoding: [0x48,0x8b,0x77,0x0c]
+    movq   (16+(0-4))(%rdi),%rsi
+
+//CHECK   movq  12(%rdi), %rsi
+//CHECK   encoding: [0x48,0x8b,0x77,0x0c]
+    movq   (16+0)-1+1-2+2-3+3-4+4-5+5-6+6-(4)(%rdi),%rsi