Re-apply r302416: [ARM] Clear the constant pool cache on explicit .ltorg directives

Re-applying now that PR32825 which was raised on the commit this fixed up is now known to have also been fixed by this commit.

Original commit message:
    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.

    This makes the output of the testcase match what binutils gas does
    (prior to this patch, it would fail to assemble).

    Differential Revision: https://reviews.llvm.org/D32847

llvm-svn: 303540

include/llvm/MC/ConstantPools.h

@@ -63,6 +63,8 @@ public:
 
   // Return true if the constant pool is empty
   bool empty();
+
+  void clearCache();
 };
 
 class AssemblerConstantPools {
@@ -86,6 +88,7 @@ class AssemblerConstantPools {
 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);
 

lib/MC/ConstantPools.cpp

@@ -57,6 +57,10 @@ const MCExpr *ConstantPool::addEntry(const MCExpr *Value, MCContext &Context,
 
 bool ConstantPool::empty() { return Entries.empty(); }
 
+void ConstantPool::clearCache() {
+  CachedEntries.clear();
+}
+
 //
 // AssemblerConstantPools implementation
 //
@@ -98,6 +102,13 @@ void AssemblerConstantPools::emitForCurrentSection(MCStreamer &Streamer) {
   }
 }
 
+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) {

lib/Target/ARM/MCTargetDesc/ARMTargetStreamer.cpp

@@ -38,6 +38,7 @@ const MCExpr *ARMTargetStreamer::addConstantPoolEntry(const MCExpr *Expr, SMLoc
 
 void ARMTargetStreamer::emitCurrentConstantPool() {
   ConstantPools->emitForCurrentSection(Streamer);
+  ConstantPools->clearCacheForCurrentSection(Streamer);
 }
 
 // finish() - write out any non-empty assembler constant pools.

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