[PATCH] D32847: [ARM] Clear the constant pool cache on explicit .ltorg directives

[Martin Storsjö via Phabricator via llvm-commits]

mstorsjo created this revision.
Herald added subscribers: inglorion, mehdi_amini, aemerson.

Multiple ldr pseudoinstructions with the same constant value will reuse the same constant pool entry. However, if the constant pool is explicitly flushed with a .ltorg directive, we should not try to reference constants in the previous pool any longer, since they may be out of range.

This fixes assembling hand-written assembler source which repeatedly loads the same constant value, across a binary size larger than the pc-relative fixup range for ldr instructions (4096 bytes). Such assembler source already uses explicit .ltorg instructions to emit constant pools with regular intervals. However if we try to reuse constants emitted in earlier pools, they end up out of range.


https://reviews.llvm.org/D32847

Files:
  include/llvm/MC/ConstantPools.h
  lib/MC/ConstantPools.cpp
  lib/Target/ARM/MCTargetDesc/ARMTargetStreamer.cpp
  test/MC/ARM/ltorg-range.s


Index: test/MC/ARM/ltorg-range.s
===================================================================
--- /dev/null
+++ test/MC/ARM/ltorg-range.s
@@ -0,0 +1,27 @@
+@ RUN: llvm-mc -triple armv7-unknown-linux-gnueabi -filetype obj -o - %s \
+@ RUN:   | llvm-objdump -d - | FileCheck %s
+
+        ldr r0, =0x01020304
+@ CHECK: ldr
+        .ltorg
+@ CHECK: 0x01020304
+        ldr r0, =0x01020304
+        ldr r0, =0x01020304
+        ldr r0, =0x01020304
+@ CHECK: ldr
+@ CHECK: ldr
+@ CHECK: ldr
+        .ltorg
+@ CHECK: 0x01020304
+    .rep 1028
+        .word 0
+    .endr
+@ CHECK: 0x00000000
+
+        ldr r0, =0x01020304
+@ CHECK: ldr
+        .ltorg
+@ CHECK: 0x01020304
+    .rep 1028
+        .word 0
+    .endr
Index: lib/Target/ARM/MCTargetDesc/ARMTargetStreamer.cpp
===================================================================
--- lib/Target/ARM/MCTargetDesc/ARMTargetStreamer.cpp
+++ lib/Target/ARM/MCTargetDesc/ARMTargetStreamer.cpp
@@ -38,6 +38,7 @@
 
 void ARMTargetStreamer::emitCurrentConstantPool() {
   ConstantPools->emitForCurrentSection(Streamer);
+  ConstantPools->clearCacheForCurrentSection(Streamer);
 }
 
 // finish() - write out any non-empty assembler constant pools.
Index: lib/MC/ConstantPools.cpp
===================================================================
--- lib/MC/ConstantPools.cpp
+++ lib/MC/ConstantPools.cpp
@@ -57,6 +57,10 @@
 
 bool ConstantPool::empty() { return Entries.empty(); }
 
+void ConstantPool::clearCache() {
+  CachedEntries.clear();
+}
+
 //
 // AssemblerConstantPools implementation
 //
@@ -98,6 +102,13 @@
   }
 }
 
+void AssemblerConstantPools::clearCacheForCurrentSection(MCStreamer &Streamer) {
+  MCSection *Section = Streamer.getCurrentSectionOnly();
+  if (ConstantPool *CP = getConstantPool(Section)) {
+    CP->clearCache();
+  }
+}
+
 const MCExpr *AssemblerConstantPools::addEntry(MCStreamer &Streamer,
                                                const MCExpr *Expr,
                                                unsigned Size, SMLoc Loc) {
Index: include/llvm/MC/ConstantPools.h
===================================================================
--- include/llvm/MC/ConstantPools.h
+++ include/llvm/MC/ConstantPools.h
@@ -63,6 +63,8 @@
 
   // Return true if the constant pool is empty
   bool empty();
+
+  void clearCache();
 };
 
 class AssemblerConstantPools {
@@ -86,6 +88,7 @@
 public:
   void emitAll(MCStreamer &Streamer);
   void emitForCurrentSection(MCStreamer &Streamer);
+  void clearCacheForCurrentSection(MCStreamer &Streamer);
   const MCExpr *addEntry(MCStreamer &Streamer, const MCExpr *Expr,
                          unsigned Size, SMLoc Loc);
 


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