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ARM assembler: fix undefined behaviour in fits_shifter

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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}
This commit is contained in:
hans 2015-03-03 19:13:30 -08:00 committed by Commit bot
parent 3709b9254e
commit 721fdb56e0
1 changed files with 2 additions and 1 deletions
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3
src/arm/assembler-arm.cc
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@ -1011,7 +1011,8 @@ 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;