[llvm] [PowerPC][MC] Diagnose out of range branch fixups (PR #165859)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-backend-powerpc

Author: Nikita Popov (nikic)

<details>
<summary>Changes</summary>

Currently, out-of-range branch fixups will be silently miscompiled. GNU as will instead print an "operand out of range" error instead.

Check that the branch target fixups fit into the proper range and have low zero bits. The implementation is inspired by SystemZ: https://github.com/llvm/llvm-project/blob/0ed8e66f88b689c152245d6b968a06fa8e67b51f/llvm/lib/Target/SystemZ/MCTargetDesc/SystemZMCAsmBackend.cpp#L31

I'm not sure if there is any way to test the absolute cases. It looks like something like `beqa 0, 0x10000` already gets rejected during parsing.

(My actual interest here is not actually assembly usage, but LLVM failing to use the correct branch kind for reasons I've not tracked down yet. Currently this just silently truncates the branch target instead of producing an error.)

---
Full diff: https://github.com/llvm/llvm-project/pull/165859.diff


2 Files Affected:

- (modified) llvm/lib/Target/PowerPC/MCTargetDesc/PPCAsmBackend.cpp (+23-2) 
- (added) llvm/test/MC/PowerPC/fixup-out-of-range.s (+43) 


``````````diff
diff --git a/llvm/lib/Target/PowerPC/MCTargetDesc/PPCAsmBackend.cpp b/llvm/lib/Target/PowerPC/MCTargetDesc/PPCAsmBackend.cpp
index 04b886ae74993..7b69e73ea65be 100644
--- a/llvm/lib/Target/PowerPC/MCTargetDesc/PPCAsmBackend.cpp
+++ b/llvm/lib/Target/PowerPC/MCTargetDesc/PPCAsmBackend.cpp
@@ -25,7 +25,26 @@
 #include "llvm/Support/ErrorHandling.h"
 using namespace llvm;
 
-static uint64_t adjustFixupValue(unsigned Kind, uint64_t Value) {
+static uint64_t adjustFixupValue(MCContext &Ctx, const MCFixup &Fixup,
+                                 unsigned Kind, uint64_t Value) {
+  auto checkBrFixup = [&](unsigned Bits) {
+    int64_t SVal = int64_t(Value);
+    if ((Value & 3) != 0) {
+      Ctx.reportError(Fixup.getLoc(), "branch target not a multiple of four (" +
+                                          Twine(SVal) + ")");
+      return;
+    }
+
+    // Low two bits are not encoded.
+    Bits += 2;
+    if (!isIntN(Bits, Value)) {
+      Ctx.reportError(Fixup.getLoc(), "branch target out of range (" +
+                                          Twine(SVal) + " not between " +
+                                          Twine(minIntN(Bits)) + " and " +
+                                          Twine(maxIntN(Bits)) + ")");
+    }
+  };
+
   switch (Kind) {
   default:
     llvm_unreachable("Unknown fixup kind!");
@@ -37,10 +56,12 @@ static uint64_t adjustFixupValue(unsigned Kind, uint64_t Value) {
     return Value;
   case PPC::fixup_ppc_brcond14:
   case PPC::fixup_ppc_brcond14abs:
+    checkBrFixup(14);
     return Value & 0xfffc;
   case PPC::fixup_ppc_br24:
   case PPC::fixup_ppc_br24abs:
   case PPC::fixup_ppc_br24_notoc:
+    checkBrFixup(24);
     return Value & 0x3fffffc;
   case PPC::fixup_ppc_half16:
     return Value & 0xffff;
@@ -202,7 +223,7 @@ void PPCAsmBackend::applyFixup(const MCFragment &F, const MCFixup &Fixup,
   MCFixupKind Kind = Fixup.getKind();
   if (mc::isRelocation(Kind))
     return;
-  Value = adjustFixupValue(Kind, Value);
+  Value = adjustFixupValue(getContext(), Fixup, Kind, Value);
   if (!Value)
     return; // Doesn't change encoding.
 
diff --git a/llvm/test/MC/PowerPC/fixup-out-of-range.s b/llvm/test/MC/PowerPC/fixup-out-of-range.s
new file mode 100644
index 0000000000000..e9c4f513cceda
--- /dev/null
+++ b/llvm/test/MC/PowerPC/fixup-out-of-range.s
@@ -0,0 +1,43 @@
+# RUN: not llvm-mc -triple powerpc64le-unknown-unknown -filetype=obj %s 2>&1 >/dev/null | FileCheck %s
+
+; CHECK: error: branch target out of range (32772 not between -32768 and 32767)
+brcond14_out_of_range_hi:
+    beq 0, brcond14_target
+    .space 0x8000
+
+brcond14_target:
+    blr
+
+; CHECK: error: branch target out of range (-32772 not between -32768 and 32767)
+brcond14_out_of_range_lo:
+    .space 0x8004
+    beq 0, brcond14_out_of_range_lo
+
+; CHECK: error: branch target not a multiple of four (5)
+brcond14_misaligned:
+    beq 0, brcond14_misaligned_target
+    .byte 0
+
+brcond14_misaligned_target:
+    blr
+
+; CHECK: error: branch target out of range (33554436 not between -33554432 and 33554431)
+br24_out_of_range_hi:
+    b br24_target
+    .space 0x2000000
+
+br24_target:
+    blr
+
+; CHECK: error: branch target out of range (-33554436 not between -33554432 and 33554431)
+br24_out_of_range_lo:
+    .space 0x2000004
+    b br24_out_of_range_lo
+
+; CHECK: error: branch target not a multiple of four (5)
+br24_misaligned:
+    b br24_misaligned_target
+    .byte 0
+
+br24_misaligned_target:
+    blr

``````````

</details>


https://github.com/llvm/llvm-project/pull/165859