Fix GHCi on Arm (#10375).

Arm has two instruction sets, Arm and Thumb, and an execution mode for each.
Executing Arm code in Thumb mode or vice-versa will likely result in an
Illegal instruction exception.

Furthermore, Haskell code compiled via LLVM was generating Arm instructions
while C code compiled via GCC was generating Thumb code by default. When
these two object code types were being linked by the system linker, all was
fine, because the system linker knows how to jump and call from one
instruction set to the other.

The first problem was with GHCi's object code loader which did not know
about Thumb vs Arm. When loading an object file `StgCRun` would jump
into the loaded object which could change the mode causing a crash after
it returned. This was fixed by forcing all C code to generate Arm
instructions by passing `-marm` to GCC.

The second problem was the `mkJumpToAddr` function which was generating
Thumb instructions. Changing that to generate Arm instructions instead
results in a working GHCi on Arm.

Test Plan: validate on x86_64 and arm

Reviewers: bgamari, austin, hvr

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1323

GHC Trac Issues: #10375

4 files changed, 23 insertions, 10 deletions

diff --git a/aclocal.m4 b/aclocal.m4
index c17a7d512b..d19a66e79a 100644
--- a/aclocal.m4
+++ b/aclocal.m4
@@ -565,10 +565,18 @@ AC_DEFUN([FPTOOLS_SET_C_LD_FLAGS],
         $3="$$3 -D_HPUX_SOURCE"
         $5="$$5 -D_HPUX_SOURCE"
         ;;
-    arm*linux*       | \
-    aarch64*linux*   )
-        # On arm/linux, aarch64/linux, arm/android and aarch64/android, tell
-        # gcc to link using the gold linker.
+    arm*linux*)
+        # On arm/linux and arm/android, tell gcc to generate Arm
+        # instructions (ie not Thumb) and to link using the gold linker.
+        # Forcing LD to be ld.gold is done in FIND_LD m4 macro.
+        $2="$$2 -marm"
+        $3="$$3 -fuse-ld=gold -Wl,-z,noexecstack"
+        $4="$$4 -z noexecstack"
+        ;;
+
+    aarch64*linux*)
+        # On aarch64/linux and aarch64/android, tell gcc to link using the
+        # gold linker.
         # Forcing LD to be ld.gold is done in FIND_LD m4 macro.
         $3="$$3 -fuse-ld=gold -Wl,-z,noexecstack"
         $4="$$4 -z noexecstack"
diff --git a/compiler/ghci/ByteCodeItbls.hs b/compiler/ghci/ByteCodeItbls.hs
index 5cc537a21a..fee50ee5db 100644
--- a/compiler/ghci/ByteCodeItbls.hs
+++ b/compiler/ghci/ByteCodeItbls.hs
@@ -219,17 +219,17 @@ mkJumpToAddr dflags a = case platformArch (targetPlatform dflags) of
                  , fromIntegral ((w64 `shiftR` 32) .&. 0x0000FFFF) ]
 
     ArchARM { } ->
-        -- Generates Thumb sequence,
+        -- Generates Arm sequence,
         --      ldr r1, [pc, #0]
         --      bx r1
         --
         -- which looks like:
         --     00000000 <.addr-0x8>:
-        --     0:       4900        ldr    r1, [pc]      ; 8 <.addr>
-        --     4:       4708        bx     r1
+        --     0:       00109fe5    ldr    r1, [pc]      ; 8 <.addr>
+        --     4:       11ff2fe1    bx     r1
         let w32 = fromIntegral (funPtrToInt a) :: Word32
-        in Left [ 0x49, 0x00
-                , 0x47, 0x08
+        in Left [ 0x00, 0x10, 0x9f, 0xe5
+                , 0x11, 0xff, 0x2f, 0xe1
                 , byte0 w32, byte1 w32, byte2 w32, byte3 w32]
 
     arch ->
diff --git a/compiler/llvmGen/LlvmCodeGen/Ppr.hs b/compiler/llvmGen/LlvmCodeGen/Ppr.hs
index 5dd27ab33b..97e4393674 100644
--- a/compiler/llvmGen/LlvmCodeGen/Ppr.hs
+++ b/compiler/llvmGen/LlvmCodeGen/Ppr.hs
@@ -52,7 +52,7 @@ moduleLayout = sdocWithPlatform $ \platform ->
         $+$ text "target triple = \"x86_64-linux-gnu\""
     Platform { platformArch = ArchARM {}, platformOS = OSLinux } ->
         text "target datalayout = \"e-p:32:32:32-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:64:128-a0:0:64-n32\""
-        $+$ text "target triple = \"arm-unknown-linux-gnueabi\""
+        $+$ text "target triple = \"armv6-unknown-linux-gnueabihf\""
     Platform { platformArch = ArchARM {}, platformOS = OSAndroid } ->
         text "target datalayout = \"e-p:32:32:32-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:64:128-a0:0:64-n32\""
         $+$ text "target triple = \"arm-unknown-linux-androideabi\""
diff --git a/rts/Linker.c b/rts/Linker.c
index 343719a9b5..68f1f59dd4 100644
--- a/rts/Linker.c
+++ b/rts/Linker.c
@@ -5569,6 +5569,11 @@ do_Elf_Rel_relocations ( ObjectCode* oc, char* ehdrC,
 #ifdef arm_HOST_ARCH
          // Thumb instructions have bit 0 of symbol's st_value set
          is_target_thm = S & 0x1;
+
+         if (is_target_thm)
+            errorBelch( "Symbol `%s' requires Thumb linkage which is not "
+                        "currently supported.\n", symbol );
+
          T = sym.st_info & STT_FUNC && is_target_thm;
 
          // Make sure we clear bit 0. Strictly speaking we should have done