[X86] Default to -x86-pad-for-align=false to drop assembler differenc…

…e with or w/o -g

Fix PR48742: the D75203 assembler optimization locates MCRelaxableFragment's
within two MCSymbol's and relaxes some MCRelaxableFragment's to reduce the size
of a MCAlignFragment.  A -g build has more MCSymbol's and therefore may have
different assembler output (e.g. a MCRelaxableFragment (jmp) may have 5 bytes
with -O1 while 2 bytes with -O1 -g).

`.p2align 4, 0x90` is common due to loops. For a larger program, with a
lot of temporary labels, the assembly output difference is somewhat
destined. The cost seems to overweigh the benefits so we default to
-x86-pad-for-align=false until the heuristic is improved.

Reviewed By: skan

Differential Revision: https://reviews.llvm.org/D94542

llvm/lib/Target/X86/MCTargetDesc/X86AsmBackend.cpp

@@ -109,7 +109,7 @@ cl::opt<unsigned> X86PadMaxPrefixSize(
     cl::desc("Maximum number of prefixes to use for padding"));
 
 cl::opt<bool> X86PadForAlign(
-    "x86-pad-for-align", cl::init(true), cl::Hidden,
+    "x86-pad-for-align", cl::init(false), cl::Hidden,
     cl::desc("Pad previous instructions to implement align directives"));
 
 cl::opt<bool> X86PadForBranchAlign(
@@ -957,6 +957,9 @@ void X86AsmBackend::finishLayout(MCAssembler const &Asm,
   if (!X86PadForAlign && !X86PadForBranchAlign)
     return;
 
+  // The processed regions are delimitered by LabeledFragments. -g may have more
+  // MCSymbols and therefore different relaxation results. X86PadForAlign is
+  // disabled by default to eliminate the -g vs non -g difference.
   DenseSet<MCFragment *> LabeledFragments;
   for (const MCSymbol &S : Asm.symbols())
     LabeledFragments.insert(S.getFragment(false));

llvm/test/MC/X86/align-via-padding-corner.s

@@ -1,4 +1,4 @@
-  # RUN: llvm-mc -mcpu=skylake -filetype=obj -triple x86_64-pc-linux-gnu %s -x86-pad-max-prefix-size=5 | llvm-objdump -d - | FileCheck %s
+  # RUN: llvm-mc -mcpu=skylake -filetype=obj -triple x86_64-pc-linux-gnu %s -x86-pad-max-prefix-size=5 -x86-pad-for-align=1 | llvm-objdump -d - | FileCheck %s
 
 
   # The first test check the correctness cornercase - can't add prefixes on a

llvm/test/MC/X86/align-via-padding.s

@@ -1,4 +1,4 @@
-# RUN: llvm-mc -mcpu=skylake -filetype=obj -triple x86_64-pc-linux-gnu %s -x86-pad-max-prefix-size=5 | llvm-objdump -d --section=.text - | FileCheck %s
+# RUN: llvm-mc -mcpu=skylake -filetype=obj -triple x86_64-pc-linux-gnu %s -x86-pad-max-prefix-size=5 -x86-pad-for-align=1 | llvm-objdump -d - | FileCheck %s
 
 # This test file highlights the interactions between prefix padding and
 # relaxation padding.

llvm/test/MC/X86/align-via-relaxation.s

@@ -1,11 +1,25 @@
-# RUN: llvm-mc -mcpu=skylake -filetype=obj -triple x86_64-pc-linux-gnu -x86-pad-max-prefix-size=0 %s | llvm-objdump -d --section=.text - | FileCheck %s
+# RUN: llvm-mc -mcpu=skylake -filetype=obj -triple x86_64-pc-linux-gnu -x86-pad-max-prefix-size=0 %s | llvm-objdump -d - | FileCheck %s --check-prefix=NOPAD
+# RUN: llvm-mc -mcpu=skylake -filetype=obj -triple x86_64-pc-linux-gnu -x86-pad-max-prefix-size=0 -x86-pad-for-align=1 %s | llvm-objdump -d - | FileCheck %s
 
 # This test exercises only the padding via relaxation logic.  The  interaction
 # etween prefix padding and relaxation logic can be seen in align-via-padding.s
 
   .file "test.c"
   .text
   .section  .text
+
+# NOPAD-LABEL: <.text>:
+# NOPAD-NEXT:     0: eb 1f           jmp 0x21 <foo>
+# NOPAD-NEXT:     2: eb 1d           jmp 0x21 <foo>
+# NOPAD-NEXT:     4: eb 1b           jmp 0x21 <foo>
+# NOPAD-NEXT:     6: eb 19           jmp 0x21 <foo>
+# NOPAD-NEXT:     8: eb 17           jmp 0x21 <foo>
+# NOPAD-NEXT:     a: eb 15           jmp 0x21 <foo>
+# NOPAD-NEXT:     c: eb 13           jmp 0x21 <foo>
+# NOPAD-NEXT:     e: 66 66 66 66 66 66 2e 0f 1f 84 00 00 00 00 00  nopw    %cs:(%rax,%rax)
+# NOPAD-NEXT:    1d: 0f 1f 00        nopl (%rax)
+# NOPAD-NEXT:    20: cc              int3
+
   # Demonstrate that we can relax instructions to provide padding, not
   # just insert nops.  jmps are being used for ease of demonstration.
   # CHECK: .text

llvm/test/MC/X86/prefix-padding-32.s

@@ -1,4 +1,4 @@
-# RUN: llvm-mc -filetype=obj -triple i386-pc-linux-gnu %s -x86-pad-max-prefix-size=15 | llvm-objdump -d --section=.text - | FileCheck %s
+# RUN: llvm-mc -filetype=obj -triple i386-pc-linux-gnu %s -x86-pad-max-prefix-size=15 -x86-pad-for-align=1 | llvm-objdump -d - | FileCheck %s
 
 # Check prefix padding generation for all cases on 32 bit x86.
 

llvm/test/MC/X86/prefix-padding-64.s

@@ -1,4 +1,4 @@
-# RUN: llvm-mc -mcpu=skylake -filetype=obj -triple x86_64-pc-linux-gnu %s -x86-pad-max-prefix-size=15 | llvm-objdump -d --section=.text - | FileCheck %s
+# RUN: llvm-mc -mcpu=skylake -filetype=obj -triple x86_64-pc-linux-gnu %s -x86-pad-max-prefix-size=15 -x86-pad-for-align=1 | llvm-objdump -d - | FileCheck %s
 
 # Check prefix padding generation for all cases on 64 bit x86.