From: Gabe Black <gblack@eecs.umich.edu>
Date: Wed, 2 Jun 2010 17:58:04 +0000 (-0500)
Subject: ARM: Don't rely on undefined behavior to get arithmetic right shift.
X-Git-Tag: stable_2012_02_02~1311
X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=14d25fbad09b3272e1d094adb1c6a298ab7b5ecd;p=gem5.git

ARM: Don't rely on undefined behavior to get arithmetic right shift.
Shifting to the right of a signed value when the MSB is one is technically
undefined behavior, even though in my experience it's done the "right thing"
and sign extended the value. This replaces the arithmetic right shift code in
ARM that uses that coincidence with some code that relies on bit math.
---

diff --git a/src/arch/arm/insts/static_inst.cc b/src/arch/arm/insts/static_inst.cc
index 41bfeac59..528111759 100644
--- a/src/arch/arm/insts/static_inst.cc
+++ b/src/arch/arm/insts/static_inst.cc
@@ -68,9 +68,9 @@ ArmStaticInst::shift_rm_imm(uint32_t base, uint32_t shamt,
             return base >> shamt;
       case ASR:
         if (shamt == 0)
-            return (int32_t)base >> 31;
+            return (base >> 31) | -((base & (1 << 31)) >> 31);
         else
-            return (int32_t)base >> shamt;
+            return (base >> shamt) | -((base & (1 << 31)) >> shamt);
       case ROR:
         if (shamt == 0)
             return (cfval << 31) | (base >> 1); // RRX
@@ -106,9 +106,9 @@ ArmStaticInst::shift_rm_rs(uint32_t base, uint32_t shamt,
             return base >> shamt;
       case ASR:
         if (shamt >= 32)
-            return (int32_t)base >> 31;
+            return (base >> 31) | -((base & (1 << 31)) >> 31);
         else
-            return (int32_t)base >> shamt;
+            return (base >> shamt) | -((base & (1 << 31)) >> shamt);
       case ROR:
         shamt = shamt & 0x1f;
         if (shamt == 0)