[PATCH] D72738: [BranchAlign] Add master --x86-branches-within-32B-boundaries flag

[Philip Reames via Phabricator via llvm-commits]

reames updated this revision to Diff 238133.
reames added a comment.

Address review comment


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D72738/new/

https://reviews.llvm.org/D72738

Files:
  llvm/lib/Target/X86/MCTargetDesc/X86AsmBackend.cpp
  llvm/test/MC/X86/align-branch-64-1a.s


Index: llvm/test/MC/X86/align-branch-64-1a.s
===================================================================
--- llvm/test/MC/X86/align-branch-64-1a.s
+++ llvm/test/MC/X86/align-branch-64-1a.s
@@ -1,11 +1,15 @@
 # Check only fused conditional jumps, conditional jumps and unconditional jumps are aligned with option --x86-align-branch-boundary=32 --x86-align-branch=fused+jcc+jmp
-# RUN: llvm-mc -filetype=obj -triple x86_64-unknown-unknown --x86-align-branch-boundary=32 --x86-align-branch=fused+jcc+jmp %p/Inputs/align-branch-64-1.s | llvm-objdump -d  - > %t1
-# RUN: FileCheck --input-file=%t1 %s
+# RUN: llvm-mc -filetype=obj -triple x86_64-unknown-unknown --x86-align-branch-boundary=32 --x86-align-branch=fused+jcc+jmp %p/Inputs/align-branch-64-1.s | llvm-objdump -d  - | FileCheck %s
+
+# Check that -x86-branches-within-32B-boundaries matches the above command line
+# RUN: llvm-mc -filetype=obj -triple x86_64-unknown-unknown --x86-align-branch-boundary=32 --x86-align-branch=fused+jcc+jmp %p/Inputs/align-branch-64-1.s -o %t1
+# RUN: llvm-mc -filetype=obj -triple x86_64-unknown-unknown --x86-branches-within-32B-boundaries %p/Inputs/align-branch-64-1.s -o %t2
+# RUN: cmp %t1 %t2
 
 # Check no branches is aligned with option --x86-align-branch-boundary=0
-# RUN: llvm-mc -filetype=obj -triple x86_64-unknown-unknown --x86-align-branch-boundary=0 --x86-align-branch=fused+jcc+jmp %p/Inputs/align-branch-64-1.s | llvm-objdump -d  - > %t2
-# RUN: llvm-mc -filetype=obj -triple x86_64-unknown-unknown %p/Inputs/align-branch-64-1.s | llvm-objdump -d  - > %t3
-# RUN: cmp %t2 %t3
+# RUN: llvm-mc -filetype=obj -triple x86_64-unknown-unknown --x86-align-branch-boundary=0 --x86-align-branch=fused+jcc+jmp %p/Inputs/align-branch-64-1.s | llvm-objdump -d  - > %t3
+# RUN: llvm-mc -filetype=obj -triple x86_64-unknown-unknown %p/Inputs/align-branch-64-1.s | llvm-objdump -d  - > %t4
+# RUN: cmp %t3 %t4
 
 # CHECK: 0000000000000000 foo:
 # CHECK-COUNT-3:      : 64 89 04 25 01 00 00 00          movl    %eax, %fs:1
Index: llvm/lib/Target/X86/MCTargetDesc/X86AsmBackend.cpp
===================================================================
--- llvm/lib/Target/X86/MCTargetDesc/X86AsmBackend.cpp
+++ llvm/lib/Target/X86/MCTargetDesc/X86AsmBackend.cpp
@@ -94,6 +94,14 @@
                    "indirect indicates indirect jumps."),
     cl::location(X86AlignBranchKindLoc));
 
+cl::opt<bool> X86AlignBranchWithin32BBoundaries(
+    "x86-branches-within-32B-boundaries", cl::init(false),
+    cl::desc(
+        "Align selected instructions to mitigate negative performance impact "
+        "of Intel's micro code update for errata skx102.  May break "
+        "assumptions about labels corresponding to particular instructions, "
+        "and should be used with caution."));
+
 class X86ELFObjectWriter : public MCELFObjectTargetWriter {
 public:
   X86ELFObjectWriter(bool is64Bit, uint8_t OSABI, uint16_t EMachine,
@@ -119,8 +127,21 @@
   X86AsmBackend(const Target &T, const MCSubtargetInfo &STI)
       : MCAsmBackend(support::little), STI(STI),
         MCII(T.createMCInstrInfo()) {
-    AlignBoundary = assumeAligned(X86AlignBranchBoundary);
-    AlignBranchType = X86AlignBranchKindLoc;
+    if (X86AlignBranchWithin32BBoundaries) {
+      // At the moment, this defaults to aligning fused branches, unconditional
+      // jumps, and (unfused) conditional jumps with nops.  Both the
+      // instructions aligned and the alignment method (nop vs prefix) may
+      // change in the future.
+      AlignBoundary = assumeAligned(32);;
+      AlignBranchType.addKind(X86::AlignBranchFused);
+      AlignBranchType.addKind(X86::AlignBranchJcc);
+      AlignBranchType.addKind(X86::AlignBranchJmp);
+    }
+    // Allow overriding defaults set by master flag
+    if (X86AlignBranchBoundary.getNumOccurrences())
+      AlignBoundary = assumeAligned(X86AlignBranchBoundary);
+    if (X86AlignBranch.getNumOccurrences())
+      AlignBranchType = X86AlignBranchKindLoc;
   }
 
   bool allowAutoPadding() const override;


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