cmd/dist: make GOARM detection better compatible with thumb toolchain

Fixes #4557.

R=dave, rsc
CC=golang-dev
https://golang.org/cl/6946078

diff --git a/src/cmd/dist/arm.c b/src/cmd/dist/arm.c
index ed640899087f32a514034897c407cde60c404730..eabf97b9ea24c5ba5749efa9b442cf716eb7f7bf 100644 (file)
--- a/src/cmd/dist/arm.c
+++ b/src/cmd/dist/arm.c
@@ -29,8 +29,19 @@ useVFPv3(void)
 {
        // try to run VFPv3-only "vmov.f64 d0, #112" instruction
        // we can't use that instruction directly, because we
-       // might be compiling with a soft-float only toolchain
-       __asm__ __volatile__ (".word 0xeeb70b00");
+       // might be compiling with a soft-float only toolchain.
+       //
+       // some newer toolchains are configured to use thumb
+       // by default, so we need to do some mode changing magic
+       // here.
+       // We can use "bx pc; nop" here, but GNU as(1) insists
+       // on warning us
+       // "use of r15 in bx in ARM mode is not really useful"
+       // so we workaround that by using "bx r0"
+       __asm__ __volatile__ ("mov r0, pc");
+       __asm__ __volatile__ ("bx r0");
+       __asm__ __volatile__ (".word 0xeeb70b00"); // vmov.f64 d0, #112
+       __asm__ __volatile__ (".word 0xe12fff1e"); // bx lr
 }
 
 static void
@@ -39,7 +50,18 @@ useVFPv1(void)
        // try to run "vmov.f64 d0, d0" instruction
        // we can't use that instruction directly, because we
        // might be compiling with a soft-float only toolchain
-       __asm__ __volatile__ (".word 0xeeb00b40");
+       //
+       // some newer toolchains are configured to use thumb
+       // by default, so we need to do some mode changing magic
+       // here.
+       // We can use "bx pc; nop" here, but GNU as(1) insists
+       // on warning us
+       // "use of r15 in bx in ARM mode is not really useful"
+       // so we workaround that by using "bx r0"
+       __asm__ __volatile__ ("mov r0, pc");
+       __asm__ __volatile__ ("bx r0");
+       __asm__ __volatile__ (".word 0xeeb00b40"); // vomv.f64 d0, d0
+       __asm__ __volatile__ (".word 0xe12fff1e"); // bx lr
 }
 
 #endif