[Driver][X86] Add -mpad-max-prefix-size

Summary:
The option `-mpad-max-prefix-size` performs some checking and delegate to MC option `-x86-pad-max-prefix-size`. This option is designed for eliminate NOPs when we need to align something by adding redundant prefixes to instructions, e.g. it can be used along with `-malign-branch`, `-malign-branch-boundary` to prefix padding branch.

It has similar (but slightly different) effect as GAS's option `-malign-branch-prefix-size`, e.g. `-mpad-max-prefix-size` can also elminate NOPs emitted by align directive, so we use a different name here. I remove the option `-malign-branch-prefix-size` since is unimplemented and not needed. If we need to be compatible with GAS, we can make `-malign-branch-prefix-size` an alias for this option later.

Reviewers: jyknight, reames, MaskRay, craig.topper, LuoYuanke

Reviewed By: MaskRay, LuoYuanke

Subscribers: annita.zhang, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D77628

clang/include/clang/Driver/Options.td

@@ -2187,7 +2187,8 @@ def malign_branch_EQ : CommaJoined<["-"], "malign-branch=">, Group<m_Group>, Fla
   HelpText<"Specify types of branches to align">;
 def malign_branch_boundary_EQ : Joined<["-"], "malign-branch-boundary=">, Group<m_Group>, Flags<[DriverOption]>,
   HelpText<"Specify the boundary's size to align branches">;
-def malign_branch_prefix_size_EQ : Joined<["-"], "malign-branch-prefix-size=">, Group<m_Group>;
+def mpad_max_prefix_size_EQ : Joined<["-"], "mpad-max-prefix-size=">, Group<m_Group>, Flags<[DriverOption]>,
+  HelpText<"Specify maximum number of prefixes to use for padding">;
 def mbranches_within_32B_boundaries : Flag<["-"], "mbranches-within-32B-boundaries">, Flags<[DriverOption]>, Group<m_Group>,
   HelpText<"Align selected branches (fused, jcc, jmp) within 32-byte boundary">;
 def mfancy_math_387 : Flag<["-"], "mfancy-math-387">, Group<clang_ignored_m_Group>;

clang/lib/Driver/ToolChains/Clang.cpp

@@ -2078,17 +2078,16 @@ static void addX86AlignBranchArgs(const Driver &D, const ArgList &Args,
     CmdArgs.push_back("-mllvm");
     CmdArgs.push_back(Args.MakeArgString("-x86-align-branch=" + AlignBranch));
   }
-  if (const Arg *A =
-          Args.getLastArg(options::OPT_malign_branch_prefix_size_EQ)) {
+  if (const Arg *A = Args.getLastArg(options::OPT_mpad_max_prefix_size_EQ)) {
     StringRef Value = A->getValue();
     unsigned PrefixSize;
-    if (Value.getAsInteger(10, PrefixSize) || PrefixSize > 5) {
+    if (Value.getAsInteger(10, PrefixSize)) {
       D.Diag(diag::err_drv_invalid_argument_to_option)
           << Value << A->getOption().getName();
     } else {
       CmdArgs.push_back("-mllvm");
-      CmdArgs.push_back(Args.MakeArgString("-x86-align-branch-prefix-size=" +
-                                           Twine(PrefixSize)));
+      CmdArgs.push_back(
+          Args.MakeArgString("-x86-pad-max-prefix-size=" + Twine(PrefixSize)));
     }
   }
 }

clang/test/Driver/x86-malign-branch.c

@@ -18,14 +18,11 @@
 // TYPE-ERR: invalid argument 'foo' to -malign-branch=; each element must be one of: fused, jcc, jmp, call, ret, indirect
 // TYPE-ERR: invalid argument 'bar' to -malign-branch=; each element must be one of: fused, jcc, jmp, call, ret, indirect
 
-/// Test -malign-branch-prefix-size=
-// RUN: %clang -target x86_64 -malign-branch-prefix-size=0 %s -c -### 2>&1 | FileCheck %s --check-prefix=PREFIX-0
-// PREFIX-0: "-mllvm" "-x86-align-branch-prefix-size=0"
-// RUN: %clang -target x86_64 -malign-branch-prefix-size=5 %s -c -### 2>&1 | FileCheck %s --check-prefix=PREFIX-5
-// PREFIX-5: "-mllvm" "-x86-align-branch-prefix-size=5"
-
-// RUN: %clang -target x86_64 -malign-branch-prefix-size=6 %s -c -### 2>&1 | FileCheck %s --check-prefix=PREFIX-6
-// PREFIX-6: invalid argument
+/// Test -mpad-max-prefix-size=
+// RUN: %clang -target x86_64 -mpad-max-prefix-size=0 %s -c -### 2>&1 | FileCheck %s --check-prefix=PREFIX-0
+// PREFIX-0: "-mllvm" "-x86-pad-max-prefix-size=0"
+// RUN: %clang -target x86_64 -mpad-max-prefix-size=15 %s -c -### 2>&1 | FileCheck %s --check-prefix=PREFIX-15
+// PREFIX-15: "-mllvm" "-x86-pad-max-prefix-size=15"
 
 /// Test -mbranches-within-32B-boundaries
 // RUN: %clang -target x86_64 -mbranches-within-32B-boundaries %s -c -### 2>&1 | FileCheck %s --check-prefix=32B
@@ -34,6 +31,6 @@
 /// Unsupported on other targets.
 // RUN: %clang -target aarch64 -malign-branch=jmp %s -c -### 2>&1 | FileCheck --check-prefix=UNUSED %s
 // RUN: %clang -target aarch64 -malign-branch-boundary=7 %s -c -### 2>&1 | FileCheck --check-prefix=UNUSED %s
-// RUN: %clang -target aarch64 -malign-branch-prefix-size=15 %s -c -### 2>&1 | FileCheck --check-prefix=UNUSED %s
+// RUN: %clang -target aarch64 -mpad-max-prefix-size=15 %s -c -### 2>&1 | FileCheck --check-prefix=UNUSED %s
 // RUN: %clang -target aarch64 -mbranches-within-32B-boundaries %s -c -### 2>&1 | FileCheck --check-prefix=UNUSED %s
 // UNUSED: warning: argument unused

clang/test/Driver/x86-malign-branch.s

@@ -6,8 +6,8 @@
 // RUN: %clang -target x86_64 -malign-branch=fused,jcc,jmp %s -c -### %s 2>&1 | FileCheck %s --check-prefix=TYPE
 // TYPE: "-mllvm" "-x86-align-branch=fused+jcc+jmp"
 
-// RUN: %clang -target x86_64 -malign-branch-prefix-size=5 %s -c -### 2>&1 | FileCheck %s --check-prefix=PREFIX
-// PREFIX: "-mllvm" "-x86-align-branch-prefix-size=5"
+// RUN: %clang -target x86_64 -mpad-max-prefix-size=5 %s -c -### 2>&1 | FileCheck %s --check-prefix=PREFIX
+// PREFIX: "-mllvm" "-x86-pad-max-prefix-size=5"
 
 // RUN: %clang -target x86_64 -mbranches-within-32B-boundaries %s -c -### 2>&1 | FileCheck %s --check-prefix=32B
 // 32B: "-mllvm" "-x86-branches-within-32B-boundaries"