[PATCH] D23161: [ELF][MIPS] Produce a correct and complete set of MIPS ELF header flags

[Simon Atanasyan via llvm-commits]

atanasyan added inline comments.

================
Comment at: ELF/Writer.cpp:1242-1262
@@ +1241,23 @@
+       Symtab<ELFT>::X->getObjectFiles()) {
+    uint32_t NewFlags = F->getObj().getHeader()->e_flags;
+    if (NewFlags & EF_MIPS_PIC)
+      // PIC code is inherently CPIC and may not set CPIC flag explicitly.
+      NewFlags |= EF_MIPS_CPIC;
+    if (ResFlags == 0) {
+      ResFlags = NewFlags;
+      continue;
+    }
+    uint32_t NewPic = NewFlags & (EF_MIPS_PIC | EF_MIPS_CPIC);
+    uint32_t ResPic = ResFlags & (EF_MIPS_PIC | EF_MIPS_CPIC);
+
+    // Check PIC / CPIC flags compatibility.
+    if (NewPic && !ResPic)
+      warning("linking non-abicalls code with abicalls file: " + F->getName());
+    if (!NewPic && ResPic)
+      warning("linking abicalls code with non-abicalls file: " + F->getName());
+
+    if (!(NewPic & EF_MIPS_PIC))
+      ResFlags &= ~EF_MIPS_PIC;
+    if (NewPic)
+      ResFlags |= EF_MIPS_CPIC;
+
----------------
ruiu wrote:
> I cannot find a simple rule for this code. What is the expected output for what inputs? Is there a documentation to describe the rule behind it?
> I cannot find a simple rule for this code. What is the expected output for what inputs? Is there a documentation to describe the rule behind it?

As usual there is no documentation on this code. The general idea - ELF flags define ISA (R_MIPS_32, R_MIPS_32R2, R_MIPS_64 ...), ABI (R_MIPS_O32, R_MIPS_ABI2 ...) and features flags (R_MIPS_NAN2008, R_MIPS_PIC ...). Some kinds of ISAs are compatible with each other and we need to select the "highest" from input ISAs. For example, if one input file has R_MIPS_32 ISA and another one has R_MIPS_32R2 ISA, we need to put R_MIPS_32R2 into the output file. SOme kinds of ISAs are incompatible. We should reject attempts to link R_MIPS_32R2 and R_MIPS_32R6 ISAs together. The same logic is applicable to ABI and some features flags like R_MIPS_NAN2008 and R_MIPS_FP64. Other features flags like EF_MIPS_NOREORDER, EF_MIPS_MICROMIPS etc can be just merged.

As to the R_MIPS_PIC and R_MIPS_CPIC flags - PIC code is inherently CPIC and may not set CPIC flag explicitly. But it is a good practice to set it in the output file so we do that. If we merge PIC and non-PIC code, output file cannot be considered as position independent so we clear PIC flag if as soon as we see non-PIC input file. If input file does not have PIC not CPIC flags that means that this file does not follow abicalls convention and we show a warning if linker mix abicalls and non-abicalls files.


Repository:
  rL LLVM

https://reviews.llvm.org/D23161