Cherry-pick "ARM assembler: fix undefined behaviour in fits_shifter"

It appears that some change to the toolchain and/or compiler flags in
AOSP master has caused this UB to be a problem for webview, causing
crashes in V8 on ARM (it wasn't an issue when using the L MR1 build
config/toolchain). Cherrypick the trivial UB fix, which avoids the
crashes.

> Bit-shifts have undefined behaviour if the shift amount is greater
> or equal to the width of the type.
>
> In this case the code would do imm32 >> 32 when rot == 0.
>
> A newer version of Clang unrolled the loop, optimized the first
> iteration away, causing the test suite to fail with:
>
>   #
>   # Fatal error in ../src/arm/assembler-arm.cc, line 1212
>   # Check failed: !rn.is(ip).
>   #
>
> as well as crashing when running Chromium tests on Android (at least
> we think this was the cause, see the bug).
>
> BUG=463436, 444089
> LOG=Y
>
> Review URL: https://codereview.chromium.org/979633002
>
> Cr-Commit-Position: refs/heads/master@{#26974}

(cherry picked from commit 721fdb56e0fc92c662c7d8b42be8a1d689c3b535)

Bug: 20064008

1 files changed, 2 insertions, 1 deletions

diff --git a/src/arm/assembler-arm.cc b/src/arm/assembler-arm.cc
index 17bf4f93a..364ccfc13 100644
--- a/src/arm/assembler-arm.cc
+++ b/src/arm/assembler-arm.cc
@@ -991,7 +991,8 @@ static bool fits_shifter(uint32_t imm32,
                          Instr* instr) {
   // imm32 must be unsigned.
   for (int rot = 0; rot < 16; rot++) {
-    uint32_t imm8 = (imm32 << 2*rot) | (imm32 >> (32 - 2*rot));
+    uint32_t imm8 =
+        rot == 0 ? imm32 : (imm32 << 2 * rot) | (imm32 >> (32 - 2 * rot));
     if ((imm8 <= 0xff)) {
       *rotate_imm = rot;
       *immed_8 = imm8;